Chris,
First off, Sealey is geared towards the motor trade, and when looking at machinery like this, they are just box shifters.
They are selling to an audience that most times have no idea of the cost of machinery, just buy it in and stick it on the bench, and as such Sealey just put a figure on it and hope it sells. They are renowned for being very expensive for machinery compared to normal outlets. Clarke (Machine Mart) is another of such outlets. Your mini lathe was most probably bought by someone who didn't know which way to stand up, hence the very little use it had had. But they do usually have a higher quality control on the items they do sell, and will be made to about the best standards they can be (but that doesn't mean set up spot on, just made). Your lathe attests to that, when measured up, it was almost perfect in all areas.
The SM 25, is a rather large machine, and is called in general terms a mill/drill. It is about twice the size and weight of the X2 you are looking at. Normal machine retailer cost is just under 1K squid.
Where you can score if buying a low mileage used machine second hand, are the bits that come with it. For starters, if you buy a new machine, you will also be required to buy a collet set and a vice, which will most probably set you back 150 squid, on top of what you have just paid for the new machine. Then you will have cutters and other ancilliaries like parallels to get as well.
If you can get all these thrown in with the second hand machine, it then becomes a very tempting package indeed. If it doesn't come with the tooling, it looks a little on the high cost side, and again, you will have to factor in the cost of essential tooling. You will need to ask the seller the right questions.
The X2 does have a few little niggles, but third party upgrades can take care of those, at extra cost to you of course. You could buy a brand new machine, put a bit too heavy a cut on, and within five minutes you can have a breakdown when the gears shed their teeth. Spares are readily available and reasonably cheap, but being without a machine for a few days can be a bit offputting, so people usually go for a belt drive conversion, which seems to cure the problem and makes the machine a lot quieter, and again, more cash out of the moth pit in your wallet.
These are all things that have to be looked at. A machine is just a collection of parts for a price, getting it to a stage where it will do the job is another matter. A rule of thumb, cost of machine = X, cost of getting the bits to make it useable will cost X divided by 2. So a 400 squid new machine will require another 200 squid (minimum) for operational bits.
John