Sounds good so far and just parts that are right the issue. Been quite a while since I totally did up a couple just for fun cars - Summer drivers and stored Winters type.
So - if going to have this apart and fitting parts I still suggest staying with, rather keeping it R-12. Pay the price which is diddle vs the project and a much better chance of lasting IMO.
Costs? Can't say. A source for possible places that will redo parts you have I've used for ages - all items not just for A/C anything.
Check for info @http://www.bakersauto.com/
They specialize in Lincolns back to 1961 - all parts. The 88 Mark may share stuff you may need. Worth a contact and probably sent on to another.
Doesn't matter but that's based in Putnam, CT but deal with the whole world.
I personally would avoid PAG oils if you do go 134a just because they are hygroscopic (absorbs moisture and can turn acidic) when exposed to air so would use Ester instead. Low mile use, "Sunday" driver shouldn't be any problem.
Totally lost with brands and who currently is making or rebuilding the higher quality stuff of anything as it changes frequently.
The costs again: If a lot redone by you or specially made up $2K would be nothing to get to. I just warn or worried that going for best prices always if quality is compromised could just ruin the project only to start all over again?
Good luck - ask who is making stuff, has NOS stuff if you plan to keep this car indefinitely,