Vor
Keep in mind this is MIN equip for IFR not the most you can stuff in the panel or everything you want. I can't disagree with Paul F. Dye above, but I will take the opposite side of the coin, going MIN (cost, weight and complexity). Of course going MIN also minimizes utility, but at least to an acceptable level for the mission's you intend to fly. On the other hand who would not want a GNS 530 /480.
Mickey you hit the NAIL. IFR equipment is EXPENSIVE. It is not a trivial add on. The extra antennas and wires cause more weight, increased drag (due to weight or external antennas) and of course $$$. How much IFR and what kind of IFR are you going to fly. GPS carries the cost burden of NAV updates.
Consider going ALL VFR equip as you describe and just add a stand alone nav like:
VAL all in one IFR unit
Another standalone idea is a used Escort VOR. Remember they had those LED segmented display head with the VOR/LOC/COM receiver incorporated in one unit. However a used out of production radio may not make as much sense. You mention the SL-30 with a CDI head as "compromise", which also makes sense from a minimalist and $$$ stand point, while getting good utility. Narco also made several all-in-on heads with everything up to GS/LOC/VOR/MB, like the Val but with analog display, which I like. They are found used and new as they are still in production. Got to look at the wallet and ask yourself how much IFR am I going to do?
HERE is my logic for going MIN IFR (like Mickey) and HOW:
Look we all know what the required FLIGHT instruments are, there are spelled out in detail in the FAR's. The controversy is if a experimental EFIS counts. I am not going to make a comment about that or step into that debate.
Again like EFIS above there is discussion of what GPS is legal and what is a legal installation. Again I am not going to step into that debate.
However I will comment on min NAV, and what one item can give you a whole lot of legal capability with out spending $1000's for equipment and $100's a year for electronic NAV data base updates. Also keep in mind there is nothing wrong with backing your situational awareness with a non-IFR approved GPS.
Bare min IFR, regarding NAVIGATION: The FAR's are delightfully Vague. All you need is NAV equipment you plan on using for the intended flight. Clearly in the lower 48, USA, you can take off (many times dead reckoning) to vectors or intercept a radial off a VOR. You could happily fly coast to coast, across the USA on the airway system and make a VOR approach where the VOR is off the field or the FAF is defined but two VORs. You can switch back and forth to get the intersection. Of course if you have LOC freq's and a simple MB, you can do LOC approaches to even lower MINS. Also ATC can call the FAF for you many times. Of course there are ASR/PARs approaches. I did a lot of IFR in a basic Piper Tomahawk with just one VOR/LOC and a basic six pack elect/vac instruments. Later I got a MB receiver and that opened up all the LOC approaches which are almost always found with an ILS. Now would I do hard ball IFR, that way? NO
So all you need is one VOR receiver/head. Most VOR's also come with LOC freq's. TO use a LOC you usually need a ADF or MB at least to ID the OM/FAF. ATC with surveillance radar at that airport can call the OM or FAF for you. Obviously even a hand held VFR only Garmin has IFR approach way points, although illegal to use it exclusively its a great tool as a back-up.
With a portable GPS, basic EFIS (or traditional flight instruments) and just one VOR/LOC (may be a MB), you can file IFR (if that is the only NAV you intend on using).
Also you are not stuck with expensive GPS NAV data base updates. Just buy paper charts/plates as needed. To buy, install and maintain an IFR GPS cost $1000's of dollars. A used ESCORT NARCO VOR/LOC head is not expensive. You can also go full meal deal and buy the VAL head with GS/LOC/VOR/MB all in one. Who needs an IFR GPS. Don't get me wrong I want one but it is expensive, and updates are a burden to remain legal to exclusively use it for IFR. I don't plan on flying that much IFR in my RV. As an airline pilot I fly IFR enough, and we do use nav data bases in the flight management computer, but we still use Paper. With a few paper enroute charts and approach plates, the OLD school VOR works.
NOW let me tell you my mission. I don't plan on filing IFR routinely. If I am flying local I am not going to go IFR for the fun of it. Cross country I will try to fly VFR, but there are times it will be nice (safer and legal) to file or POP-UP. Examples are say a marine layer or a little FOG in the morning grounds you (for VFR), but near by weather and airports are VFR; you could file and with a short IMC climb get on top to VFR conditions and on your merry way. Now lets say your en-route, VFR and find the broken under cast is now starting to become solid. The destination still has high ceilings and good Vis. You are enjoying a good tail wind, and instead of ducking down low to finish the flight, you continue at altitude. Now you need an IFR let down on an airway. POP-UP and get it, let down to a VFR conditions to the airport. If you had to, you could fly a VOR or LOC approach.
Staying really current may be a pain in you bare min IFR plane. I fly IFR at work so it's not an issue for me. You can (should) fly and practice attitude instrument flying skills (scan-x/check-interpret-control) with a safety pilot in VFR conditions. To me key and the foundation to all IFR operations and procedures is the scan and control. You can practice procedures on a home PC. However GA simulators are getting very good. A good 1-2 hour SIM session with a good instructor in a full GA Sim can be a great workout and cover situations, weather, winds and emergency scenarios you may not experience in a year of IFR flying. Also they can turn the performance up to match the RV. Flying IFR in a C-172 may not be a good transfer of knowledge and skill to the RV. We all agree I am sure single pilot IFR in a RV means an autopilot on board.
With "IFR" flight instruments one VOR can get you a lot of capability on the cheap. Obviously an IFR GPS is the only way to really go, but it comes at a premium price. The unit will cost several thousand ($3K to $8K not uncommon) and the data base will need maintenance. I don't know all the legal and operational issues with IFR GPS in general aviation, because I never have owned or used one. When I was flight instructing, IFR GPS where just coming in, not common. Approaches where overlay only and not stand alone at the time.
George