What are you using the slide for? theres two things you can achieve with it- more capacity for wider cuts and trenching cuts- being able to cut grooves across the timber rather than cut straight through.
If it’s only larger timbers then it’s better value to find a saw with a bigger blade rather than a slider. Even the priciest sliding saws can have flex and slop in the rails and runners that can effect accuracy.
There’s plenty of good quality 250mm ish non-sliding saws on the market. With a bit of shopping around you’ll find Ines with 305mm blades. Nothing to really go wrong with them so worth considering second hand ones. My dewalt dw713 has been on the go for nearly 10 years and I hardly ever need anything bigger. I’ve also recently bought one of the all bells and whistles Kapex saws but still revert to the dewalt 99% of the time.
If you are insistent on sliding then buy from somewhere you can actually get your hands on the machine. Price isn’t really a guide as to how stiff and accurate the mechanism is. Some cheap ones are actually pretty good, some expensive ones are pretty bad. Because the sliding mechanism can wear I’d be less keen to buy one second hand.
In the showroom drop the blade down and slide the saw right out on its rails and give the handle a wiggle and see how much the blade moves side to side.