400 sqft. of decking is not very much. I am not sure how big your yard is, but get more decking than you think you need if you can. I have 1100 sqft of decking and wish I had more. My back yard is 1/2 acre though. Some stats from my pool and price since I put in mine 1 year ago for comparison.

1100 sqft washed aggregate decking
Free form pool, 3.5ft*5ft*4ft, 20,000 gallons (36 ft long, width varies 24 ft at widest, 16 ft narrowest, I think 100-110 perimeter)
coping is flagstone
20 ft long mossrock waterfall, highest is about 4 ft tall
matching mossrock planter on opposite side of pool
tanning ledge (4 inches deep)
Tahoe Blue Pebble Tec surface (gives a lagoon look)
Jandy LED color lights (2 of them)
2 hp pump for main circulation
1 hp pump for waterfall
Polars 280 cleaner with booster pump
Jandy Aqua Pure Salt System
Price was $31,000

I am sure there has been price increases over the last year. I would certainly entertain multiple companies and multiple pool plans. There is always somethink you wish you did differently.

Good Luck and have a great Memorial Day.

Mike