I plan on replacing the stock batteries with DieHard Automotive Battery, Platinum P-2 - Group Size 65 when they fail. Im looking into also installing a 735X150 Solargizer to the current ones and then the replacments when that time comes.
My question is I would like to have a 3rd battery on the blazer for running odds and ends as needed. .
Having an extra battery NOT connected to your starting batteries can be real nice to have. I used my diesel Blazer for a few years to run power tools. This where I was building a cabin and no grid-electricity was available. I started out with one extra deep-cycle battery, but later made it two - hooked up to a 3000 watt (6000 watt surge) inverter for AC power. Also used a 120 watt solar panel. I'd run heavy power tools all day long, with no problems. I'd usually drive the Blazer once to town mid-day, and that 10 mile trip was all it took to fully recharge the storage battery.
Regardless if running power tools, or just watching a TV, or whatever. You can install a low-draw, full-time rated charge relay for $20. Wire it so . . . when the ignition is on - the starting batteries and the extra storage battery are all connected to each other and charge whenever the engine is running. With the key off- the storage battery is isolated, and you can't accidentally kill your starting batteries.
One added feature to the system is a cut-off switch to that relay. This way, if I've been using the storage battery a lot, I can start the engine first- let the starting batteires recharge, before that relay energizes. Otherwise, as soon as the key is "on", a rush of starting battery power goes to the storage battery in an effort to charge it.
As to buying batteries? I'd stick with an auto battery (non-commercial) whenever possible. for starting batteries Warrantees are usually much better and this is just about always cheaper in the long-run.
For normal useage, paying double or triple for an AGM battery is a huge waste of money. With normal use they rarely last any longer then conventional batteries - but you CAN run them upside down if you find that to be important??
At present, there are only around three companies making all the batteries in the USA. Sears Diehards are made by Johnson Controls (same as Walmart Maxx, Interstate, Caterpillar, etc.). Sears AGM batteries are Optima verisons - also made by Johnson Controls.
If you've got a big storage battery, a 20 watt solar panel can make a max of around 1 amp of charge current in full sun when the sun is directly overhead.
Here in NY, a 20 watt panel will rarely make even 1/2 amp. That's why I use a 120 amp panel when parked. It usually makes 3-4 amps when the sun is out and overhead [- and that only happens for a few hours on those rare sunny days around here.
Any solar panel larger then 20 watts needs a cheap controller. They can be found for $30.