1) It is very heavy on memory usage and CPU both. It's been pushing the 2gb addressing limit for a while, and the larger the maps get the more cpu cycles it eats up.
2) Yes, but it can be modified to be as high as you want. The default setting is 2000 fleet points, which in most cases will translate to 300-400 ships based on your make-up. There are also low/high fleet settings when starting a game, for a 25% penalty or bonus to fleet points, respectively.
3) Can't help ya there. 
4) Well you can't play Entrenchment without vanilla Sins anyway, so you'd have to get it one way or the other. After that if you like it and want to add on Entrenchment's features it'll just be $10. It'll still be in beta for a while, so should give you plenty of time to decide.
5) Most of the posts for the past couple of weeks are for the Entrenchment beta. Anything about mines, starbases especially 