Minus the advertising, I'd suggest you to look into the size of unique/ unbanned (depending on what you're going to need your proxies for) proxies. This is a much more important factor that simply the size of IP pool, because, well quite a few companies (not pointing any fingers) use the size of their IP pool purely for marketing reasons. Well, technically they're not lying, but they're not saying the true number of proxies you're getting.
Can't comment much on 2 and 3, as it depends, but speaking about no. 4, well, it solely relies on what you consider as "cheap"