I'm on a well also, slightly different aquifer though.
Check pH of your water from the tap, use the tub faucet as it does not aerate the water like the sink faucets do. You may find pH in the 6 ish range..
This will duplicate the source water your feed for the RO/DI unit will receive, so it is a valid test.
Aerate your sample for 10-15 minutes and duplicate. Chances are good pH will end up high 7's, low 8's. If this is the case, you'll need a bubble stone and air pump in your makeup water pretty much 24/7.
I have phosphates in my well water now, did not 3 yrs ago. I use a homemade GFO and sediment prefilter upstream of my RO/DI unit. I got the filter housings from an RV supply as that was where I saw them first, however you can get a cartridge type filter @ the home center and many wal-marts. The inlet opening under the filter housing is not threaded, but you can get a rigid plastic tube that will fit from the autoparts places.
force the tubing into the housing and cut to fit short of the bottom of the cartridge housing, and fill with media. Cut a 1"x1" piece of screen fabric and place over the return side opening to keep out larger pieces.
Unthread the hose fittings and replace with appropriate size piping and terminate with a 1/4 inch compression fitting. Use a 1/4" refrigerator icemaker kit hose to connect to the inlet of the RO/DI unit and your done.
As for the booster pump, there are some other options prior to dropping the big bucks, a smaller bladder tank like the ones used for an undercounter water filter/dispenser will help even out the pressure surges when the pump cycles. (Handy but not required) You can adjust up the pressure at the well, or you can hit the local harbor freight for a $100 booster pump.
I go the cheapest route, I wait till laundry time, shower time, lawn watering time, etc. to make water. The pump cycles more frequently to keep up with demand, so you have shorter intervals of lesser pressure, making more water in the same or shorter time period.