Two big disadvantages and a concern surround buying an RV that's completed:
1. No repairmans certificate for me.
2. They cost a lot more then a used Cherokee for a modest increase in performance.
The concern is that if I don't build it myself, I have to rely on the builder doing everything right and the A&P that gave the initial signoff for the AW certificate being legit. I only list this last thing as a concern because I figure it's unlikely to be a big deal, but it's something I think about.
There's actually a company here in Eugene, OR (Synergy Air) that, if I understand correctly, would assist in converting parts and money into airplane while keeping the builder involved enough to be legit, but I can't imagine it's very cheap. If I build, I'd definately attend some of their builders classes, but unless I'm willing to do the real work for myself, it seems like it sorta defeats the purpose of getting to build my own plane.
Obviously, I need to do more research, but that's where I am now. Maybe the thing to do is to buy a certed plane to get my instrument rating in, then start slowly building a plane. If I already have a plane and build an RV incrementally, then I can still fly. I build everything I can except the powerplant, then sell the certed plane to pay for my new Eggenfellner H6 & EFIS, spend a month of furious work finishing it all up, and then start flying my homebuilt.

In the meantime, I'm thinking about which signoff to get next. I'm thinking it might not be a bad idea to plan my training as follows:
1. Sign off in Cessna 172
2. Get Complex in 172RG
3. Begin instrument training
4. During instrument training, get my tailwheel endorsement in their instrument rated Aeronca Champ to save $$$.
5. After instrument checkride, save up for my multi-engine. I heard that if you get your multi-engine before you get your instrument rating, then you're only certified for VFR ME but instrument SE. Someone else said the smart thing to do was to get your instrument training, do multi training at the end, and combine the instrument and multi-checkride to save $$$.