Maybe I can help with some Eco info...
This thread is 14 pages long but has tons of Eco info from several guys doing conversions right now...
http://www.shoptalkforums.com/viewtopic.php?t=69767
As far as dollars spent, $500 would probably get you a good used Eco, another $2k for everything to make it run. That gets you 170hp. For $600-700 you could add a non-intercooled turbo ( if you built the kit yourself ) and would get you to probably 225hp. If you were gonna go any farther, you need to build it for boost, i.e. turbo pistons, better rods, billet crank. Then you could go
big turbo, intercooler, big boost. You could easily make the 400hp but it won't be happy on pump gas and you will seriously stretch that $5k you want to spend...
The big thing is how cheap you can find an Eco to start with and how much fabrication and salvage picking you are willing to do. A lot of the turbo bits could be had on the cheap but decent pistons, crank and rods will be retail items. Trying to run 15-20psi boost without the bottom end parts, you'd be building a grenade.