propane heater replacement

our heater needs to be replaced. I've gotten some quotes and they seem to be all over the place. We have a 28,000 gallon inground gunite , kidney shaped pool. The first from our local pool company, which I've lost confidence in, was $4000 for a 400K btu hayward propane heater. This included electrical hookup but no gas hookup.

over the weekend I picked up some calcium at a local shop and they gave me a quote of $2700 for a hayward 300K btru heater...obviously not an apples to apples comparison.

I guess the first thing I have to do is figure out how many btus I need. It occurs to me that I can mail order the heater, get an electrician in to hook it up and I guess a plumber to hook up the propane. Is that the best way to do it?

tia,