Long attempt at helping-
IF money is an object, you could get lots of utility from a basic, used IFR TSO 129 non-precision approach, non-WAAS GPS Navigator.
Reliability, updating, need for an annunciator switching set and other considerations stack up poorly against more modern production. It could last years or just hours, no warranty or repair ability. Your EFIS database, if any, is like an IPAD, good for awareness, but not IFR navigation filing. Without being able to shoot a GPS non precision approach, you may be very limited in training and checkride. You still need a VOR or ILS or localizer, ground based destination alternate filing approach navaid without a WAAS navigator, but a 129 non-precision approach box gets you DME and NDB substitution in the US.
Best $/hour overall may be to train and take the IFR check at the proper time in a certified plane with the most modern panel you can afford, but moreover, with a compatible CFII. Will you really shoot IFR in IMC in your RV? You surely can setup the RV to file IFR, climbout and get through some IMC weather with just a VOR. A single EFIS and single NAV radio is not leaving a lot of outs and redundancies, at any experience level.
Edit- your home airport lists only NDB and RNAV GPS Instrument Approach Procedures-
https://www.airnav.com/airport/KDMO
RNAV (GPS) RWY 18, RNAV (GPS) RWY 36
NDB RWY 18 and NDB RWY 36
KRCM has a VOR, but needs DME, so that's out.
KVER has a vanilla VOR, but it is a VOR-A meaning circling, only getting you to 1200' above the airport.
KMHL and KRAW only have RNAV and NDB.
A bunch of airports nearby, but only KSZL has an ILS /LOC. If they allow civil training to low missed approaches, you could do a checkride in the 30nm local area, I think, but not optimally at all. The SZL approaches need RAPCON or DME, if allowed to be there (takes a civil landing permit to touchdown, but some bases allow approach work). IF the standards allow, an ILS and localizer approach at SZL and the VOR-A at KVER could be a checkride completion.
On paper, assuming you do all the work/wiring, A TSO129 GPS unit, antenna, tray and annumciator/switch unit (you can build your own) can be "just" a few thousand $. Installing, certing- many, many hours and huge cost unless you do the labor. It may last for years, or may fail soon. Updating databases may be tedious and as pricey as a new LPV/WAAS navigator. You could install a DME or NDB, but that is not cheap or seen often.
For the VAL, some report not great reception range, take all best measures with the antennae/cabling/setup installation. Jesse Saint or Stein advice is proven, as is finding others results with the actual units you are considering or bought.
You would need both an instructor and examiner willing to fly in your plane. If radar vectors to the ILS and LOC at SZL are allowed on the checkride, then you fly the full VOR-A circle, ok, but in this day that is a compromised method of attack.
I'm sure there is a segment of current instrument pilots that fly near state of the art IFR, IMC at work or play, but not in their actual RVs for a myriad of reasons. Maybe as many as fly awesome IFR RV panels. Having your IFR ticket and some real experience flying IFR in good weather and finding your personal mins BEFORE doing so in your RV may be the true economical decision point.
One uncertified EFIS and Nav receiver is about as cheap as it comes to meet the mins of FILING IFR and touching IMC weather. It is also potentially very limiting. It may suit you fine for years, but I don't think a stranger can tell you if it will work out. A CFII that knows your flying, if you have a true pattern/style, would really help.
My RV-6 was built, not by me, as steam then single GRT EFIS, SL-30 nav/comm, similar to your plan, but with a 2 axis Trutrak. It may stay that way forever. I added a second PFD in a Gemini and spruced up the essential bus/endurance.
I'm a 20 year, full time IFR pilot though, military and civil and current ATP, I know my limits, and in a way invested many times the price of the RV in arriving to it. The plane CAN be “cheap”-ish, but the journey to it should be as savvy as possible to make it “economical” to your capes, needs or dreams.