Pricing at tirerack.com for Firestone Transforce HT 16" LT225/75-16 E at 2680 lbs capacity is 151 bucks a tire times 4 equal 604 bucks.
Pricing for four 6 lug 16" white steel modular wheels at trailerpart.com is 42.95 times 4 = 171 bucks.
Thats 775 bucks for about the best tire and wheel set up for a trailer out here. My trailer equipment dealer/mfg sells wheels at those prices and one of my area tire dealers will match tirerack.com pricing if asked.
You don't need a 16" spare as your current spare works fine. I always use the 15" ST spare. Its never rolled on the road.