105 is nice stuff. If it's properly setup it will shift just as well as pricier ultegra. A lot of the difference in the groupo is the STI shifters. Since you're building a tri bike that issue goes away as you'll be using bar-end shifters.
I've been racing the Planet X 50/82 combo for a year now and it is fast. The extra $1500 to go to Zipp is just not worth it. Here's the clincher, or tubular in this case, don't skimp on tires. I went with Conti GP4000s. At $100 a piece they are expensive, but the ride quality is amazing.
Do frame and groupset first. The flexibility on seating position and overall fit will do more for your ride than the wheels will.
I'm operating on a limited budget and want to know what is worth splashing out on and what isn't; should i go fully shimano 105 on the groupset, or should i buy 105 with an Ultegra crankset, etc.
105 is plenty. Microshift shifters, or the old stand by DA bar ends (does shimano make a 105 equivalent?). Not terribly expensive. I'd bet RD/CHAIN/CASSETTE/FD/CRANKSET/SHIFTERS can get pieced together on ebay for ~300
Also, how much of a bonus will a Zipp 808/Disc configuration give me over, say, Planet X 82/101mm wheels?
Very little, some aero is better than no aero wheel (if you can afford it), but aero vs more aero is diminishing returns. Save the $ and get an aero helmet (if you don't have one), will save you more than the difference in those wheels likely
Finally, should I buy the frame & groupset first, and use my current (non-aero) wheels for races, or should I first buy wheels, and use them with my current bike until I can afford the Frameset?
Frameset. Much more comfortable to ride/run afterwards on a tri bike. The wheels will gain you a handful of seconds on a 40k (maybe a minute or so, I am sure there is hard numbers out there), but a proper aero position and helmet will gain you more. (assuming you don't have clip-on bars on your road bike, this could be fairly substantial).
On the same wheel set, I am ~1mph faster on my tri bike/helmet vs my road bike over a 40k. I can stay aero the whole time, and am less sloppy with position on the TT bike vs the road bike in the hooks
Thanks a lot for any input,
Late addition. You say you are on a limited budget. You CAN modify the road bike for better TT position with forward seatpost and clipons/appropriate head gear. But if you are considering carbon aero wheels, i don't think you are REALLY on a budget, it's just there in case your wife reads this .
I currently ride 105 and I like how crisp they shift. Also agreeing with Adam here, if you are looking to go to carbon rims, Ultegra wouldnt be that much of a shock to afford over 105, but not too much difference with shifting quality.