Well 16 gauge is for topspin......18 is for people with flat strokes
The more tension put on the strings, the more control; the less tension, the more power.....middle is probably around 57-60 lbs.
Prices of strings are very cheap. The very common and cheap synthetic gut (probably what the pros give you) is around 4 dollars. Hopefully your pro shop isn't ripping you off by tacking a crappy labor fee on there.
As for brands stick with Gamma, Prince, Wilson, Head, Big Banger, Kirschbaum, Babolat. I may have left a few out.

It's kind of odd that you give us your height in the American system and give us your weight in the SI system. Weird....