I bought my Troybilt Horse model in 1978. Yes I have had to put new tines on it and a new engine, but it goes on and on. I grow a big garden. This is more tiller than you need for a 20 x 20, but sure a nice machine for a little more space.
I just dug out the invoice on it. In 1978 I paid $652. Gee that was 38 years ago. 652/38 = $17.16 per year. Add on a new engine and tine replacements and it is still an inexpensive investment considering what I have got out of it over the years.
No matter what machine you may buy, it is mandatory that you check it over before each use and tighten the bolts, and change the oil now and then. If you are to keep any machine running, it needs maintenance. Plan on it.
If that is too much bother, buy a shovel.
Gardening at 5000 feet elevation, zone 4/5 Northern Utah, Frost free from May 25 to September 8 +/-