Cost is the major factor. Without using stock components (IE, TBI, because it is the ONLY way to cheaply adapt factory parts to a non-corporate motor) you will NOT be able to do this I would say for anything under $2000.
I looked at this for Oldsmobile, and you'll be in the same boat with the Buick. Fuel rails, intakes, and mating that all up with a throttle body, is big $$ for non-corporate (IE stuff GM didn't fuel inject) motors. A 454 throttle body, bolted to a TBI to Q-jet adapter, is going to be amazingly cheap comparatively. No, it won't be port injection, but that's where the cost vs. benefit comes into the equation. Port is better, but it's just a lot more hassle to run if it wasn't done stock.
Check into the EBL system for the EFI control (posts here and on thirdgen.org) and you could likely have it up and running almost instantaneously. Megasquirt could probably do similar.
I'm not a big Buick guy, so I don't know how the distributor situation is for them. With the Oldsmobile, since the Olds engines lasted until 1990, there is no shortage of distributors already set up for electronic control. Doesn't matter that it was carb. You can convert the non-EFI distributors over pretty easily apparently, so it's probably a very slight stumbling block at worst.