I'm in an area with very hard water.(working on a getting a water test to quantify)
I'd like to purchase and install a system myself.
I have done many plumbing projects, but have never bought a softener. I was told by an old-time not to buy one from a box store.
What is a good quality model/brand with option to purchase online that I should look into.
Look for one with a Fleck control head. There are a number of web suppliers that offer excellent prices for complete units delivered to your home.
In deciding on a specific control valve look for the size of the internal passages in the head--typically 3/4, 1, or 1 1/4 inch for residential. A good rule of thumb is for the internal size to equal the size of your main water line--but even 3/4 inch internal valves are satisfactory for most residential applications with larger main water lines.
Most residential units have a single resin tank but Fleck makes "twin tank" systems and while more expensive because the head is more complex and they have two resin tanks they offer advangates in some circumstances--including very high hardness.
The third major variable is electronic or traditional electro-mechanical control circuitry. Electronic provides a bit more flexibility but DIY replacement of individual components is doable in the electro-mechanical. With the Electronic the circuit board is user replacable but a single unit that does virtually everything except drive the valve--that is done by a replacable motor.