I think your good with what you've got.....M3 would get slapped with your current setup!!
I know the costs as i have just done an upgrade so lets see if this helps you!!
I got a closed deck block from ebay for £300 (thats the block you want to go with as its alot stronger)
I then sent that to Alan Jeffery (engine tuner) and he done the rear thrust conversion and converted it to a 2.1 stroker with forged pistons etc!! i think that was about £2300
Then you have the option of 11mm ARP head bolts (£200 i think) or 14mm cosworth ones (£600 nas the block and heads need modified)
Then you need uprated oil pump ranging between £170-£300 i think (i went 12mm cosworth one)
Then I sent the heads off my engine down to him so he could re-condition them and fit them to my block.....with new water pump and Kevlar timing belt, gaskets etc i think that came to about £1400
I think in total just for the engine and the build etc i was about 5k.........after that i then done lots of other stuff like parallel fuel mod with braided fuel lines etc and insulated spacers etc....oil cooler......basically it can be a total money pit!!
I done all the work on my car bar the engine build so i saved money there........basically you can end up spending loads, it just depends how much power your looking for!!
Hope that helps
Jase