I think the real estate system is different in Thailand to the USA.
You could use one agent, but you will only see the properties they have on their books.
I think you would be better spending some time looking around areas and deciding a location/condo block you like first and then seeing which agents have a condo there.
Alternatively post what your requirements are (be specific and provide a budget) on a Pattaya Facebook page and agents will contact you.
Remember it is a buyers market, so offer less than the asking price.
The non o retirement visa is much easier to obtain than marriage visa( if you are over 50).
Getting married in Thailand is difficult enough without making things more complicated with a marriage visa.
Before you come to Thailand get an online 60 day tourist visa, you can then extend it for a further 30 days in Thailand. This should be plenty of time to open a bank account here, deposit the money and get the non o visa.
Opening a bank account is the most difficult part of the visa process and these days it seems you need to pay an agent to help, it costs 4 to 5,000 baht, but you can do this on the 60 day tourist visa.
simply to keep the bank up to date with my address in case they decide to post something to me. I can't remember if the documents they produce for immigration show you live in the province.
It definitely seems to be getting more difficult than it was when I arrived less than 2 years ago, but back then, with a bit of persistence and travelling around banks/branches, I managed to open an account with a tourist visa.
If you try and cannot the going rate for an agent for this task seems to be 4-5,000 and might be worth doing. The rest of the visa process is easy, so agent not required (I think you know this already).