Although it's risky to generalize, an old rear-tine (or "center-tine") tiller suitably refurbished will likely be a much better choice for the typical Texas (if there is such a thing) use than any front-tine unit you're likely to get more economically. The front-tine kind are of some use for small areas with lots of "obstrucions," as they can be a little more maneuverable; but they're not generally as good at turning the soil deep enough for "real gardens."

That assumes that the drive-train components are in reasonably good shape, and that it's a brand-name for which parts are available.

Note that tillers are much like lawnmowers, in that there are only a couple of manufacturers who produce lots of "brands" for distributors. For some parts, you can go to the manufacturer¹ even when there's no local support for the particular brand name on the machine in your area - if you can figure out who actually built the thing.

Tillers may have "belt drive" and/or "gear drive" power trains, and usually you find a combination of some sort, combining both kinds of components. Replacing a truly worn out gearbox may be the rough equivalent of buying a new tiller, but of course belt replacement is just normal maintenance. Tiller tines wear, of course, but they should be easily replaceable if they're really in bad shape.

If the owner has been needing annual tuneups for some reason there may be reason to suspect the engine is worn, but if the tuneups were "just maintenance" someone told him was a good idea that doesn't mean a lot. There generally are only a couple of engines used in virtually all brands, and fix-it parts are widely available for the common ones. "Exotic" brands may be less available.

New replacement engines, in a worst-case scenario, are generally available at fairly reasonable prices; but it would depend on the condition of other parts whether that's a reasonable thing to consider. Engines of the kind normally used on tillers can be in pretty bad shape and still deliver useful service if you pay some attention to the little quirks they develop with age. (We older folk should appreciate that.)

¹ In a tale recited in detail elsewhere at the 'cat, I coerced the manufacturer into offering me a "warranty repair" on a brand name mower - two years out of warranty (by filing a CPSC report on the brand name). The mower manufacturer of course blackmailed the engine manufacturer into giving me a new engine at no cost. When I complained that the argument between the two manufacturers deprived me of use of my mower for an entire mowing season, the engine manufacturer bribed me with a nice gym bag and a coffee-table book on the history of the company. I also got the opportunity to meet local "code enforcers" about my "overgrown lawn," so that I knew them when the neighbors complained about my son's junker van that he abandoned on my "back-40."

John