Welcome back !
I'd ditch the old diesels - even with chips their performance will be worlds away from today's stuff.keep them as sentimental value - they won't be worth much else.
Have a list of things
- you must have on the layout
- things you'd like to have.
You say 2000 to present day. There was a massive amount of change in this period.
Right at the start you still had 50/50 old diesels and the 66s and 67s coming on line.still had loco hauled cross country and vacuum braked old wagons for the engineers.
Then it changed !
Nowadays the old stuff is mainly gone, there's a handful of older diesels on test trains and charters.
Of course we know have the 68, 70, 88......
Personally I feel the earlier part of that period was way more interesting - are you going for nostalgia or does it have to be what you see on the tracks today ? There's a lot of questions I'd ask yourself before committing to using any of your cash.
Getting the baseboards built is a good idea - or there are several companies that will sell you a flat pack version such as model railway solutions, white rose modelworks , tim horn baseboards.
Have fun !