Personally, I am always considering budget as well. So I have used "opportunistic" modern groups on bikes. I let the deal dictate. So for instance, I picked up a mini-group of Ultegra 6501 (9 speed): brifters, both derailleurs, brake calipers, for a super low price. I then picked up a nice 8/9/10 speed wheelset at a garage sale at an even better price. So that became my group of choice. I recently picked up a couple of complete nine speed Dura Ace groups, one will probably end up on my next build.

If you have trading stock, you might find someone on the forum where you can trade some items to complete a group. I traded the wheelset from above with RobbieTunes, for a wheelset more appropriate for my added weight (compared to Robbie). IMHO, best way to share a deal is to trade value for value. No one really wants to pay full market for stuff (me neither!), but trading components of equivalent value is a great way for both parties to get a deal, without one person subsidizing the transaction.

I think the somewhat older Shimano groups look fine on a classic bike. I have Ultegra 6501 on two of my bikes. One has the Hollowtech II crankset, it doesn't look so vintage, but it is OK.

+1 The Dura Ace 7700 would look good as well.

And the tricolor Shimano 600 stuff looks great, and its eight speed, which means the brifters can be repaired.

The great thing about considering budget, is if you get the group right (attractive price), you can just install it. Ride the bike for a while. And when you get the urge to upgrade, no problem, as you should be able to get your money back out of the first group. So its almost like renting a group for a while (for free, if you buy right). Tough to beat that!!