The grounds for diagnosing MS is generally based on the McDonald Criteria: https://www.mstrust.org.uk/a-z/mcdonald-criteria
You can see from the chart that the most important evidence is the MRI. Your history of symptoms plus the physical examination is also used by the neurologist in coming to the diagnosis.
The lumbar puncture is generally only absolutely necessary if the relapse history or the MRI evidence is insufficient to show dissemination in time and space of lesions. But as Carole said, some neurologists like to do one anyway (belt and braces type approach). George’s information about LPs is absolutely right, the caffeine and sugar in full strength Coke is what’s needed as well as laying down. Some people don’t get the headache without following this advice, and a few get the headache in spite of it.
It might help you to do some research into the diagnosis of MS. Try looking at About MS | Multiple Sclerosis Society UK
Also, in terms of disease modifying drugs (DMDs) it does help to have some idea what might be offered to you so you could have a look at https://www.mstrust.org.uk/about-ms/ms-treatments/ms-decisions-aid before you actually have the discussion with the neurologist. DMDs are taken to reduce the number and severity of relapses, they aren’t a cure for MS or even for symptoms. They are simply to lessen damage done to your nervous system by inflammatory relapses.
You aren’t likely to get a free hand in which drug you can take, it depends on how ‘active’ your MS is, what prescribing centres have been set up in your hospital, your neurologists preference and of course, your own choice once the options are whittled down to a few drugs.
All drugs do of course have not only various effectiveness in terms of average relapse reduction rates, but also the potential for side effects. So it’s a case of weighing up the risks versus the potential benefits.
Best of luck.