A tough choice IMO... Cuz if I had a R34 GTR, I would not sell it.
Just thinking about it...
I was going to say EVO / Silvia / RX7 FC + FD / STi.
I had a chat to Aaron from Import Monster over the weekend and we discussed about his STi vs his R34 GTR.
In his opinion, the R34 GTR is kinda like the 'grew up' version / brother to the STi.
Both 4WD / AWD, Turbo Charged, 2 doors etc... But the R34 GTR was an over all larger car, more top end, which I agree.
(Stock vs Stock) In the ADM at the moment, there is the EVO X / STi / 350Z / 370Z, but I still think the R34 GTR is winner, due to having larger engines to the EVO / STi and being Turbo over N/A to the 350Z / 370Z.
Depending on the market you want to go in I suppose...
Luxury / Performance I would have gone with BMW 335i / M3 / M5 then further up a Porches 911, but that seems out of your budget.
I would have also thought about Nissan V35 / V36, which are pretty much 350Z...
Performance family sedans = 4 door V35 / V36 / HSV / FPV.
Comes down to which market you want to go into / price range / characteristics of which car you prefer to have a change to IMO.
I am kinda particular tho... If I had to change, I would like a 240Z / 260Z / 280ZX or a KGPC10.
A nice JDM cruiser like a Toyota Chaser / Nissan V35 / V36 / El Grand / President.
Any S Chassis or an AE86.