My 2 sense:
Where is the best place to find parts? (NOS, OEM & Aftermarket) Jacks Rebel Warehouse, Ebay, Craigslist, etc. I have had some cool scores on Ebay from MC shops going out of business and selling NOS. I search all the time and have had success. Parts for these bikes are not as uber-rare as most people (namely non-450 riders) convey.
What service manuals are available? (IBN please, not just the publisher) Get the Honda Factory Service Manual. Clymer's is good and has more additional info for the home mechanic, but covers other 400-450cc Hondas so you need to make sure you are referencing the proper section. Kinda easy b/c the 450 is almost its own 'species' in there and easily separated out.
Who has the best diagrams? I reference the service manual and also the exploded diagrams on MC parts websites like Bike Bandit.
What obstacles am I going to encounter that are best resolved by "just accepting it"? (For example: you are not going to find a 450 fuel tank. Buy the aftermarket 3.4 and modify the mounts.) None, don't give up your search. I have scored full running bikes for well under 1k, there's all the parts you need! Putting word out (here, FB) can get results. I have made some awesome 450 parts scores simply by checking sites like the news.
What parts DO successfully swap between the '86 250 & 450? (There seems to be varying opinions on this because one post says "nothing"; another says "only the turn signals"; and yet another says "most peripheral items but nothing frame or engine". I ask this question specifically because I am able to get a 250 in great cosmetic shape (seat, mirrors, lights, pedals, tank, etc.) but the engine is blown. Should I buy it before it's gone? No. The 250 and 450 are different bikes. They are only share the name Rebel. You can adapt parts and accessories from almost any bike to fit with some ingenuity, but there are no direct parts that swap with the 450 from the 250, maybe the handlebars, they are both 7/8"
What other valuable information did you need that you wish someone told you up front?Well....: Use the proper tools for the job-toss those vise grips, buy a torque wrench and good tools, buy a multimeter and learn how to use it for elec diagnoses, Put on a $1 in-line fuel filter, tune your bike yourself (oil, valve adj, SP's filters, etc.), do not use high octane gas (waste), measure twice cut once, read the whole procedure twice before you start, ATGATT (falling sucks, but you want to be able to get up!), do not use regular charger for lithium batteries, Keep your brake system tunes/replace pads, check/replace your chain/sprockets if worn, I use JRW for carb cleaning and starter rebuilding if needed. I think he does calipers as well, but I did mine myself. I use Spectro Semi Synthetic 20w50 in my bikes and change it ~50 miles before storage every year. Jack B. (the other Jack at JRW) educated me about the 450 liking the 20w50 oil a long time ago and ever since I started using it my bikes run better (to me). It is the only oil I will use in my 450's. I am sure I forgot a few things.
This forum (and the old Rebel250.com) is where I got a lot of my knowledge from, lot's of good advice and guidance here, welcome home my friend!
Where are you?