There is a lot of good advice in this thread but it is easy to lose sight of the forest because of the trees.
If it was me I would get the donor as cheap as possible and just get it running if it isn't already. That is gonna allow a very important thing to happen, you getting to know that particular bike (yes every bullet is probably a little different). It will cost very little to make the bike run and operate the best it can, within the stock limitations. Provided time patience and exploration of web info and identifying parts suppliers you like.
You can do a lot of things within those parameters that you will later want/need to do anyway (for example exhaust upgrade, TM carb install and tune, electronic ignition, spin on oil filter, brakes, tires, suspension, right side shift, tacho, delete electric start, correct the engine breather and so on). This will also help distribute costs over time.
I have done a fireball this way (incidentally bought new bullet for $4200, plus ~$4000 in parts, don't want to think about receipts though). Right now working on another fireball, bought junk but complete non running bike for $300, similar amount in parts as the first bike. Took it all apart as soon as I got it home. BTW it is still not running a year and a half later. Part of that is being too busy and/or laziness but the first way sure looks easier and possibly more enjoyable in hindsight.
If you want to run the bike around while Tom is working the head and Chumma is building a crank, just track down a spare head and crank assembly. Will keep your engine build down time as small as possible.