There are more roads to Rome then the traditional 4 lane one...you buy an OEM version of an old version,or second hand.He will not be designing the new space shuttle,or maybe after the ROV he will...
On this site you can find some old OEM versions that do the work just fine:
http://talk-more.biz/s-Autodesk.php
Read the FAQ to be comfortable with the way ordering works,i'm not really sure about the legality of it,but if its ok this is a pearl,use at your own risk!
Or try ebay,i found some nice advertisements,like Inventor Suite 2009 for $800...
About the pc performance:
As long as you keep you parts database under 1000 parts it will be fine,providing you have a recent pc with enough graphic memory...i got a mate running it on a laptop with a 8600M GS 512mb,runs just fine..