Heater Recommendations

We bought a house with a 25,000 gallon IG vinyl pool, with a broken heater (completely disconnected) last year. The kids and I loved the pool during our VERY hot summer, but by Sep 1st, the season was over because the water was so cold. The wife wants a heater back on the pool.

So we have a gas line installed and need to replace the heater and have the new heater hooked up. What should I be looking at? Is it worth it to buy online? The local dealer quoted me $1500 for the heater and $100 for installation - that seems like a decent price.

Where do I start?