Zoe
I've had the LP done twice. They give you a shot first to numb the area. I sat on the bed and leaned over whie they did the spinal tap. It didn't hurt but you have to be still. I had to lay flat for 4 hours before I could go home. After the second one I had a headache for a week. Not pleasant but not unbearable either. I have heard of some people who have had some problems afterward, but I think this is the exception rather than the rule.
My understanding is that none of the tests alone (spinal, MRI, evoked potentials) is definitive. It is weighing the results of all of them together. Even then it is not definitive and I have had several opinions and the Drs do not interpret the results the same.
BTW lyme will show in the spinal fluid about 5% of the time. This is one of the least accurate tests for lyme.