Yes, you can convert a tourist visa to an ED visa. However most schools will require a minimum of 45 days left on the visa in order to start the process for the ED conversion.
I could be wrong and if I am, I am sure someone will correct me, but similar to the Non-O, with the Non-OA you can apply for a 1 year extension. However this will require either 800k in a Thai bank account 2 months prior to applying or 65k+ in monthly transfers for 1 year.
if you haven't lived in Thailand for an extended period of time I would suggest getting an METV before you arrive as you can get nearly 9 months from that, then at about month 7 or 8 start making decisions on what you're doing next (longterm options such as elite or going to a different country).
it's not about being rich, it's about giving good advice... It's sound advice to tell someone to try living in the country first but it's bad advice to tell them to get an ED visa, especially when their long term plan is to get an Elite Visa.