Ok.

I'll do things bit by bit.

1. A 6 hour equivalent volume turnover is standard for commercial pools; 8 hours for residential. For your pool, 28 GPM gives you a 6 hour turn over (28gpm x 6hr x 60m = 10,800gal). Coupled with an adequate filter, 6 hour turnover will proved excellent water quality.

2. If this is your filter (90sft Star-Clear Plus Cartridge Filter), then 30 gpm is an ideal flow rate for high water quality, and long cartridge life. Cartridge life and performance is a function of (a) low enough flow, (b) cartridge quality, & (c) proper cleaning. This page covers how to clean a cartridge. And this is probably the correct replacement cartridge: Unicel C-8409 Replacement Filter Cartridge for 90 Square Foot Hayward CX900RE. If you can find a Filbur brand cartridge, that would also be OK. Avoid Pleatco and all Asian made cartridges.

3. Given the flow requirements of 30 gpm or less, a 2 speed 3/4 HP pump would be idea, but they are hard to find. If your existing pump is in good shape, you might want to install this motor on the pump, with a speed toggle switch. You'd be able to run it on low, except when vacuuming.
http://www.a1poolparts.com/-strse-23...eed/Detail.bok

If you wish to replace the pump, this one would be ideal:
http://www.a1poolparts.com/-strse-18...ent/Detail.bok

Since you are in Florida, a multi-speed pump is probably a bad idea. The electronics do not mix with with lightning strikes, and the costs of repairing the electronics seems to 75% of more of the cost of a new pump. A 2-speed pump will give you most of the savings, and much better life.

4. TDH -- Total Dynamic Head -- is a function of the resistance to flow due to pipe and valve friction. Although elevation changes come into play, they are not a major factor. Whether length is a factor, depends on flow and pipe diamenter. The TDH of 100' of 6" pipe at 30 GPM would be almost nil; with 1" pipe, it would be very high.
Here's more info: http://www.poolforum.com/pf2/showthread.php?5890

Good luck!