1. sometimes you can get minor food deposits under the denture , but this is minimised if a good denture and close fitting denture is constructed.
2. There are many implants on the market. I personally tend to use astra, beacuse I have very good success rates with these.
3. I personally would not reccommend mini implants for the upper jaw.  They may be cheaper, but they will not last as long as conventional implants.

I hope this helps

Regards

Kal