MarketPress and Authorize.net

HI,

1. How do I set up an Authorize.Net gateway for MarketPress? If it is currently not supported, what effort would be involved in my paying to get this done?
2. According to this forum post -- https://premium.wpmudev.org/forums/topic/marketpress-multisite-questions#post-96352 -- the Paypal Express gateway limits a user to purchasing from up to 10 stores in one checkout.
a. If I have to use Paypal Express for now, can I increase the number of stores that can be included in one checkout? If so, how?
b. if I use Authorize.net, is there also a limit of 10 stores?
3. If I only create one store for now (on a multi-site install), is there a limit on how many items a user can purchase in one checkout?

Thanks.