Where I live, you can get a sewing machine service for £40-£75. This means that you could buy a decent old machine and have it done up (much like buying a second-hand bike and having someone fix it). However, while most parts are pretty reasonable, if you get a very odd make or have to bad luck to break a very tricky part, repair costs could be as much as buying a new one.
In terms of sewing machines, I'd advise against buying one of the electric ones with the little display - there is more to go wrong and the maintenance is less easy. I've considered upgrading on eBay myself (still looking as I want a particular model), but would only buy one from there if described as in full working order/recently serviced because most people selling them have inherited them or whatever and don't know anything about what might be wrong. Bernina is a very good brand, but any of the common ones (Bernina, Janome, Singer...) will be alright if you're not doing a lot of heavy sewing.
Sergers (we call them overlockers) are a different kettle of fish. They can be a nightmare to thread, often requiring tweezers - I was taught to never EVER unthread one and always to cut the old threads just above the reels and tie the new threads on and winch them through by hand. If the tension is off it simply won't work and the tension wheels can get warped easily. In all seriousness, unless you are a very committed sewer (or do a lot of stretchwear!), I would not bother. You will spend more time cursing the stupid thing that unthreads the needle every few inches and just won't stay in than you will sewing happily with it. You can zig-zag the edges of fabric for the same effect or just fold the seam allowance in half and sew it like that.
If you STILL want one, then definitely get one with a four-thread option (it means you can overlock things together, not just do the seam allowances), practice threading it multiple times before you hand over the cash, and Toyota and Bernina are the best makes.