2.5 eurovan motor is a taller block so you can get more displacement, but you gotta do internals. I believe the biggest you can go on a Aan/3b/7a is right around 2.3-2.4 liters. Bang for your buck is 20vt motor with forged rods, that should be good to 700 horsepower or so, from what I've heard anyways. 7a head is better because they see less abuse, but if you have a turbo head in good shape then that's fine. I wouldn't really bother too much with head work except bigger/undercut valves, and maybe some polishing. There's a really good thread somewhere here about porting the 20v head, and there's not a lot to gain. The 3b intake supposedly flows the best, then you'd probably want a good equal length tube header. 7a cams are slightly more aggressive and flow better up top, particularly with a big turbo.
The numbers I've heard for limiting factors in the stock motor are as follows:
Rods: 400 hp
Pistons: ~700+hp
Crank: don't think I've seen anyone break one from too much power
Head: 1000+
Crank thrust bearings: 1300
If you really want to go all out I'd do the stroker block since they're cheap, he pistons/ie rods, coated bearings, arp hardware all around, polished head with lightweight valve train and oversized and or undercut valves, custom manifolds.
For the power you mentioned I'd just do rods, bearings, arp hardware, and whatever you feel like doing for flow. Basically as long as the bottom end can handle it, the rest of the motor can be pushed. Head work just means running less boost for the same power.