FYI: ALL Riding Mowers in the United States are manufactured by ONE Company, MTD.
While some are built to better specs than others, they are basically pretty much the same.
What to look for:
Heavier Sheet metal, heavier plastic parts, one big plus is a Cast Iron front axle, if they have that, then most likely they are of the better build (It will cost more, but you get better quality).
As for engines, Briggs & Stratton are the highest quality engine on the market.
Kohler & Tecumseh have more torque, but they will not last nearly as long as B&S engines. They tend to burn oil & develop leaks sooner also.
(Long story short, Get the B&S Engine with more HP, you will be glad you did in the long run).
Yes, an oil filter is a big plus, if you change oil & filter regularly, the engine will outlast the rest of the machine. The filter helps tremendously vs. No filter.
Over the past 40 years I have owned (possibly) every brand of Riding Mower sold, the best bang for the buck I can recall was a Huskee mower from Tractor Supply. Cost was mid-range, it had a 24 HP B&S engine with a 42" cut, had Hydrostatic (automatic) transaxle and a Cup holder!
In my area, people swear by John Deere.
They typically cost around double the exact same model in the store brand (new).
JD used to build their own mowers years ago, used Kohler & Tecumseh engines. They were rock solid, heavy duty mowers (though engines were "meh"), they started cheaping out before finally ceasing production.
As for deck size, get as big a deck as you can, makes for a smoother cut over a large area. (Just make sure you have enough HP to pull it!)
For 1 acre, you should go with no less than 38".
(I was "given" an older John Deere rider, 15 HP Kohler, 38" deck, in almost new condition, only had 100 hours on it. It works, but even the 15 HP is weak for the 38" deck. Uses 1/2 quart of oil every 2 mowings (~4 acres).