We understood your question pretty good I would say. xD
Not sure if you understood your own question xDDD Anyway, let me answer to that ^^
You've asked if you can install the SQL Server on a VPS or if SQL Server 2000 would run on a VPS for a Mu Server Season 4 or Season 6. xD
The answer, again, is yes. ^^
For Season 4, it should work on SQL Server Express 2008, but that depends on what SQL Server that MuOnline database was backup-ed with.
I recommend for Season 6 a SQL Server Express 2008. Karli gave you a link.
This is a new question ^^
You were asking where you can buy a VPS in the first post, not that you already have one.
Yes, Mu Server would work on Windows Server 2012.