1. Assuming it's a casting set up, 30lb braid is probably sufficient. Thin braid can lead to dig in, and thick braid doesn't cast as far. If you're pond hopping you are probably fan casting as far as you can.
2. Yes, cheap mono backing absolutely works, and also mono helps grip the spool.
3. I first started with Amazon cheap braid, but generally you get what you pay for. Cheap braid will probably fade faster (if that matters to you), could be 4 strand which is louder through the guides (again, if that matters to you), and become more limp vs taught over time. For moving baits, probably not an issue, but for bottom techniques you probably don't want limper line. I'm sure everyone has their opinion on braid, but just buy the best line you can get in your budget.