Personally I think you done well to get the price lowered. In future you could ask them to contact you first before carrying out any extra work. This always help to save the shock of a big bill.
It sounds like they've sorted out some problems for you and your car might be grand now. I'd give it a week or so to see how (if any) different it feels to drive before making any decisions to change it.
If you're interested in doing a bit of DIY I always find a Haynes manual quite useful. It'll show you how to do the easy maintenance tasks like changing the
, spark plugs etc.
If you don't feel confident enough to carry out the work yourself, ask friends and family if they know of any good mechanics. Failing that, try asking your local motor factors to recommend someone. They deal with private mechanics every day and will be able to point you in the right direction.
I hope your car goes well for you and that you don't get dis-heartened with it.