Wow, that sounds like a pretty power hungry pump.
How many GPM are you pumping? How frequently does it cut in?
Perhaps you could consider a cistern system with a smaller pump.
Also, make sure you don't have any water leaks...
I've been using "modified sine" for about 2 years now. I have 2 inverters. Small, 600 Watt for my computer, and "small things".
I plug in my "Bad Boy" inverter for running my power tools. I've even welded using inverter power.
Sometimes my computer power supply will hum. Otherwise, I have no problems running drills, saws, welder, grinders, etc on modified sine. Well, the welder was maxing out my capacity.
If you browse E-Bay, you will find several 5000 Watt Load / 10,000 Watt peak inverters, as well as a few other high power inverters (3000/6000, 3500/7000, 4000/8000, 5000/10000, 6000/12000).
Prices for the imported high power modified sine inverters are less than $500. Search for "inverter", then sort by descending price.
I'm not too impressed with some of the Chinese workmanship. Mine should be 12V in, 120V out. However, I think it is about 1V off... so, if I feed 12V in, it cuts out due to low voltage, especially if I have voltage drop due to significant load. It runs fine off of a 14V system.
I also have a 5000W 110/220V transformer, also snagged from E-Bay. I did re-wire all the electrical connections on the transformer recently. I was amazed at how cheaply it was constructed (lots of thin wire inside)... now I don't want to see the inside of my inverter.
http://cgi.ebay.com/5000W-Voltage-Converter-Step-Up-Down-Transformer_W0QQitemZ360079269900QQihZ023QQ
categoryZ50588QQrdZ1QQssPageNameZWD1VQQcmdZViewItem
Anyway, I use 220V power from the inverter+transformer for my table saw and welder.
I'd encourage caution if you choose to wire multiple inverters together as recommended above. Make sure they support that configuration. I think you are better off purchasing a native 110V (60HZ) or 220V (probably 50HZ) inverter. Running at 50HZ should be fine for most applications, except for some electrical/mechanical clocks.