Custom minimum order per group plugin development

[sotoz]

Hello,
I'm trying to develop a new free virtuemart plugin that adds the functionality that for each client/user group the admin can assign a minimum total order before the client can check out. If the user is in a group and has less than the required minimum order on his cart he will not be able to checkout and an appropriate message will be shown.
For example :

Clientgroup1 has minimum order 100Euros before they can checkout,
Clientgroup2 has minimum order 200Euros before they can checkout


I have some questions though.

1) Is it possible to do that with a simple plugin?
2) How can I use the shop groups backend form with my plugin so I can add another field there?
3) What kind of plugin this should be registered as? (calculation, shopper, custom)?
4) Any points I should pay attention to?

thank you in advance.