The problem with version 5.0 is that its not yet a stable or official release. Most web hosting companies host a number of sites on the same server and at times in the same instance of MySQL. MySQL themselves so not advise using 5.x in anything other than a test environment.
Your have 2 options as I see it. Firstly you could use a web provider that give you a dedicated machine (or virtual machine) and get them to install MySQL version 5.0, they should be willing to do this because it won't effect any other clients. However it's likely to cost you a lot of money.
The second option is to set up your own webserver and you can then install anything you like on that. But of course there are associated costs to that.
On a side issue, have you actually tried to write procedures and functions in MySQL? By day I'm an Oracle developer and we have a site built using WebDB, MySQL stored procedures are nowhere near as mature as Oracle's implementation and depending on the complexity of them it might be very difficult to replicate what you're doing in Oracle.
Check out my site which has plenty of info about stored procedures in MySQL, as I said by day I'm an Oracle developer so might be able to help if you have issues converting your Oracle code into the MySQL version.
http://www.mysqldevelopment.com
Andrew Gilfrin
http://www.mysqldevelopment.com