Different capacity SWG have different prices.
Ask them what exact brand SWG system and the capacity they are proposing and also for the scope of work entailed.
With a 25k pool, you should be looking for a SWG with a 60k capacity.
With a single speed pump, you will want to run the SWG at 100% to minimize pump run time to that needed to produce the required FC in the shortest amount of time.
Installing a VS pump, along with a SWG, will save money on electricity in the long run.
Many here have ordered online. Some have self installed and some have had a handyman or their pool guy install it.
I ordered a Circupool system and installed it myself. The warranty is valid for self installs with that brand.