For what use? The 195 has a top limit of 180A, so will likely need a dedicated 16A or 20A supply. The 151 will work off a domestic 13A socket.
The 195 has a bottom limit of 40A. Not much good for car bodywork, nor anything else under about 2mm, or 1.6mm if you are experienced. This is still a hobby welder, not a workshop jobbie.
The 151 has a lower limit of 30A, so much easier to do bodywork, probably down to around 0.8mm thickness or less, with experience.
You can get bargains on ebay, but you really either need to test the m/c, or take somebody who can. If not, you are going on the word of the buyer. You can get bargains, like when somebody buys the m/c for a project, then sells afterwards, so you know the m/c has not seen a lot of use.
New is fine if you can afford it; sometimes you can get a few quid knocked off a demo model, or go to Machine Mart with a VAT-free voucher. (Chaps sometimes give them away in the Private Forum in the Members Section, if they don't need the voucher themselves).
Mechanically and electrically the 151EN has no more problems than any other Clarke, and less than some other makes. It handles gasless wire okay if you need it outside, and is good with argon/CO2 mix, speaking from my own experience. Spares are relatively cheap, and easily obtainable. It is a sturdy little m/c, steel case, and is a model that has been going some years now.
The essential thing about owning a 151 is that you don't take too much notice of the Clarke 'manual', which has the settings wrong, for some reason. A few other Clarke welders suffer from this problem as well, apparently.
Much better to rely on the experience and experimentation of other guys on here who have the settings sussed. Put something like "Clarke 151EN settings" in the search box at the top of the page, and you'll soon find some interesting threads to read through. Here's a start: http://www.mig-welding.co.uk/forum/showthread.php?t=4971