To answer your question on PTO power, you need to consider the power requirements of the implements you are planning to use. A general rule of thumb is 5 HP per foot of width for a brush cutter or tiller, depending on your ground conditions. As long as the tractor has enough power to meet or exceed the minimum horsepower requirements of the implement, you would most likely be OK. Generally, performance will be better if you have more than the minimum power required.

Also, for some tasks such as dirt work, more weight is your friend; but if lawn mowing is your major use, lighter is better. Most implement manufacturers rate their implements by PTO HP, but not all - IIRC, King Kutter rates theirs by engine HP. PTO HP is one specification that by nature has to be measured pretty consistently between tractor manufacturers.

I would suggest that you need to consider first the jobs you need to do with your tractor, then size the implements you need to do those jobs. Once you know what implements you need, deciding on the tractor size becomes easier. As an example, my primary use for a tractor is rough mowing 12 acres of a former hay field, probably once or twice a month. I was going to pick a five foot cutter, but people more knowledgeable than I suggest it would take a long time to mow, and that a six foot mower would be more suitable to the task. As a result I wound up with a cabbed tractor with 36 PTO HP, rather than the smaller one I thought I wanted initially.