First question, is there any difference between the 3 mini and 5 mini starter sets other than price and number of figs?
The 6 mini starter packs are the better buy. They include the 3 starter set guys, a mascot, and 2 players.
Are there any guides around from Vet players for buying/expanding teams. I'd like to get 4 teams total.
Not really. Since kick off has masons & Brewers I would recommend getting some combination of:
The thing with he Union is they are mercenaries, so if you have a bunch of union your other teams can hire some onto their team and give more variety.
Who each union player works for will depend on the guild they are hired into, and you can only have 1 union player in a non-union team.
I'm not seeing a big market for goalpost markers. Can someone point me at one, or do most people just make their own?
Most people make their own, though there are a few ebay/etsy style people who 3-D print them.