100 stall pastures or barns are definitely the best, you get the most space for your money. Personally I like keeping all my permanant horses in barns and then enough pastures to grow hay for them all.

The barn manager isn't bad if you don't know how to show by looking at the horse but I make more money showing on my own than with her, even when I have it set to show only competitive horses.

If you're only showing, not breeding yet premium isn't as important but it helps a lot with riders since you can easily reassign and consolidate riders, so that's up to you really if you think it's worth it just to use those benefits.

I'm not sure of any super useful clubs for SH but I also don't breed them, ISH has Implementing Irish, TB's and WB have Thoroughbred Warmblood Society or something like that, X have a few different clubs that I use on my SA, so not one main on but plenty of benefits, KNN also have a few with discounts to a lot of the top boys, I have no experience with RID, none that I know of with AA either but since they're a pretty popular breed there are probably a few.

Picking a breed really depends on how much of a challenge you want, RID and KNN are both difficult and PONs aren't easy either.