MD80, it is hard to give advice on this without a little additional information. You mention running a brush hog and digging, but didn't mention why you want a heavy skid steer. I presume that part of the reason is that you want auxillary hydraulic flow rates suitable to run the brush hog, but not sure if there are other reasons. The really heavy skid steers aren't going to be near as responsive or fast as the smaller ones, some of which could still run a brush hog. The weight restriction you mentioned is going to put you around 6000 lbs for the skid steer (attachments vary in weight but you would probably want flexibility up to 1500 pounds at least as often it is good to transport a bucket as well as another attachment).
Also it wasn't clear when you say a tracked skid steer if you mean adding tracks to a wheeled sklid steer or actually purchasing a full time tracked skid steer, also known as a compact track loader (CTL). Your soil conditions and need for speed are again going to help dictate that choice, as the rubber tracks can wear out quickly in abrasive conditions and aren't generally as fast as their wheeled counterparts but do better in soft or moist soils better than wheeled skid steers. For personal use the wear issues probably won't be as big as issue as the purchase cost which will be a lot higher for a CTL than a wheeled skid steer loader. If you only occasionally need the benefits of tracks, then you can add those to a wheeled skid steer. I have not used them on my skid steers but have heard they can be hard on wheel bearings.
Rental units have their pros and cons- they are usually regularly maintained but also abused more than an owned piece of equipment. Owned equipment usually isn't abused as much but may not be maintained as well (just depends on the owner). Whether someone keeps maintenance records depends on how disciplined they are and is an indication of condition.
Choice of brands is pretty subjective- there are some that people will tell you to clearly avoid but the decision is probably more on the particular issues with a model you select (engine model, reliability, etc.) than the brand. I have personally only owned Bobcat brand skid steers but have used Case skid steers as well. If you go to older historic models than the brand can affect the method of steering (hand or foot control) and accessory attachment (Bobcat had the patent on the Bob Tach) but nowadays most model can have steering configured electronically like other brands and the quick attach is near universal. Parts availability and local dealer locations would factor into my decision. I personally have kept to older models that have limited electronics (because that's what I can afford to buy), but that doesn't mean that they are necessarily easier to fix.
Your idea of someone familiar with skid steers helping you look is a sound one, as they can point out issues and comment on condition. You should also do your homework on particul;ar modesl by reading posts on this board, specification charts, and model reviews. You will be a long ways up by selecting some additional basic specifications such as operating capacity and axillary hysdraulic flow rate and then look at specific models that fit those. Depending on the brush hog model, there will be a minimum at maximum flow rate. You should also look at that for other attachments you are interested in using.
Hope this helps some, and I expect others than use their skid steers commercial to add their observations or opinions as I am just a non-commercial user that also has observations working on construction projects in my professional life.