2
I'm no designer, but if it's just a matter of making blocks and colours look like they should look, then it's all in the theme. Maybe ask the designer to take a look in an existing theme's files? A lot of it is CSS, which he should know.
Regarding paying someone to do work for you, there are no problems. You can pay/get paid for doing anything with XOOPS - as long as you don't keep the code secret.
If you want a special module/theme and pay someone to develop it, that's fine - but the module/theme will have to be published and/or released when approched.
Good luck with your site.