NDB need not apply
I am a CFII and had an IFR RV-4 at one time. I never took a check ride in a RV or trained anyone for IFR ops in a RV.
I found my RV-4's IFR capability where not utilized much in the 800 hours I flew it. It did not have an autopilot so it was......... I can't say was a handful, but it was not relaxed. To do administrative duties in IMC like getting plates or en-route charts out was not fun, in part the cockpit size and part the plane is not super stable. Its not that it's bad but if you have to let go of the stick for a second and the wing does dip, speed can build quickly. I second the opinion that an autopilot for single pilot Ops is a must have. Most examiners will let you use it for brief periods on a check ride between approaches as I understand it.
As far as NAV equipment, check the archives, but you do not need a NDB. In Van's RVator news letter many years ago there was an article about flying IFR in RV's that was pretty good. It was long a go and it might be in the book that compiles back issues of The RVators.
You will need a GS/LOC/VOR/MB.
This is a pretty cool product:
http://valavionics.com/valavionicscgi/store/VALstore.pl?user_action=detail&catalogno=VAL_INS_422
Do you need two VOR's for and intersection? No but if that is they only way to ID it you need to do some fast switching or get a radar fix.
(I am ignoring IFR GPS and assuming a check ride scenario and a VFR GPS would not be allowed, even though it is a good tool for situational awareness, even IFR. Obviously the full meal deal Garmin 430/480/530 would be wonderful. If you have a panel mounted VFR GPS you might get away with leaving it on and having it give you continuous dist/track to a way point in an terminal area for general situational awareness. That is what the old NDB beacon did for you, give you a an arrow pointing to home all the time. With the modern VOR I am sure it will give you auto radial which is nice, but nothing like a GPS map.)
Single COM is OK to but again, that could be busy and tricky to monitor ATIS and ATC by switching back-n-forth. Work load goes way up. The Apollo is SL40 is cool, which allows audio monitoring of two freqs. The SL30 adds the VOR/LOC and GS to the COM 1+1/2 com.
http://www.garmin.com/products/sl30/
http://www.garmin.com/products/sl40/
As far as flight instruments that is in the FAR's.
The experimental EFIS is also another issue. My opinion is (not a FAR's), as long as you at least have the good old Needle/Ball/Airspeed backup you are good to go (electric T&B or TC, mechanical pitot/static airspeed and altimeter). There's no back-up is mandated for EFIS in GA planes, but common sense rules here. Of course an independent autopilot from the EFIS may save your bacon, since people in a stressful situation, not current on partial panel do very poorly, as the accident rate post traditional instrument vacuum failure attest to. The bottom line is the CFII and designee flight examiner need to be comfortable with it.
Good Luck George ATP/CFI/II/MEI