> I go there with the intent of getting married, do I need a special visa or not, considering I can already enter legally?
Yes, you absolutely need a visa (specifically the K1). Entering the US with the intent to get married without it is immigration fraud!
I've been through this process myself and I can tell you that it's quite the lengthy process and there's financial requirements and many checks along the way.
For me it was about 18 months from start of K1 application to issuance (delayed due to COVID) and then about 6 month wait on employment authorization (and greencard about 6 months later).
These times could be much longer if you're not from a western country or mess up the paperwork along the way (easy to do, there's quite a lot of stuff both partners need to file).
Just letting you know so you can start the process early.
There's several forums online for people going through such a process with a lot of resources, I'd suggest looking there for other's experiences.