Advice please


I am looking for advice please to help me make up my mind which route to follow, the supporter or the pay to blog route.

I have been running a very successful motorcycle rental site since 1998 which is in it’s current guise it’s running on a set of perl scripts which was knows as HotlinksSql, it has been rock solid over the years but is now suffering from massive spam attacks and I want to move the whole site over to a WPMU site, no small task as there are over a thousand advertisers on there at the moment. At the moment this is free service for everyone who has a listing, unless they want a premium listing which they have to pay for.

Going down the WPMU route I will give them a blog each within the network at say £5.00 per month, with this option they will be able to add a whole lot more content, pictures, pages and be able to have enquiry forms and of course with the plugins you offer a whole heap more if they want it.

But I am concerned about the starting point for this project, especially the sign up forms for the above mentioned plugins, can I add additional fields to collect information so that I can as the ADMIN place their blogs in the correct place in the navigation tree.

At the moment I am leaning toward the pay to blog route to start with, but would really appreciate an comments because once I start on this project I don’t want to have to start over because I have overlooked something, it’s going to be massive!

Thanks in advance for your help.