Tank - YES, great tank for one betta, definitely no other fish.
Decorations - the arch is good, but plastic plants tend to rip their fins (imagine wet tissue paper and you've got betta fins). I'd suggest silk plants. A good test for plants is to run pantyhose over them. If they snag, they'll snag betta fins. And you'd be surprised what actually snags!
Heater - you can get the preset ones from Walmart, but if you ever need to up the temp to treat some illness, you'll have to buy another. I recommend this seller on ebay: pmioutlet. Great service, and very cheap heater prices. He sells an adjustable heater
that I purchased and works great.