Upgrade needed for a 2.5 n/a! Help needed!

Hey there, i know this may be a 'blonde' question but i was curious..I currently have a 2.5 n/a in my 90 and i am looking to upgrade? although considering the rebuild i am on a budget. Is there anyway i can upgrade my 2.5 through a turbo or anything? or will a 200tdi be simpler and more cost efficient?
Thanks

If it's working OK why change it?? Especially as you say that you are on a limited budget!!