I don't know it, but I have a friend from the US who tells me it's better to get the retirement visa within the Thailand. This is because of insurance. Just something for you to look out for.
I have used Agents bunches of times and I have no complaints. However, I think it's better to not use an agent if possible. I concern that once you start with the agent you'll be stuck using the agent.
Another problem I have had is where I open my bank account. For many of your interactions with your bank, you will have to go to the branch where you opened the account. If you open an account in Bangkok then move to Samui, you may have to travel to Bangkok to get your banking paperwork... There may be ways around this but do you really want the hassle.
I would recommend coming on an METV or DTV, figure out where you want to live for the next five years and start working on the requirements to moving here (seasoned bank account, purchase/lease condo, etc etc).
For the past 10 years, off and on, I have used METV and done border bounces to get a new stamp. Yesterday I was in Cambodia for about 15 minutes to get a fresh stamp. I am starting to think it's pointless because they stamp you in for 60 days regardless if you have a visa... But I am cheap bastard so I intend to get my money's worth.
In those 10 years I have been pulled aside twice. Once in the airport once at a land crossing. They talk to you, ask you a few questions and send you on your way. This is a great opportunity to be a jerk, say something really stupid then get denied entry... However, if you can manage to be a half-way decent person, complement Thailand, Thai people and Thai food, they will likely stamp you in no problems.
AirBNB, Agoda, are two sites to check with. AirBNB aren't technically legal so you may want to use them, if you have to extend your visa, while staying there.
My agent charges a bit more to do the visa extension. It seems counter intuitative, but it is what it is.
I am also on METV. I do border-bounces every 60 days and skip the extension.
You can do both yourself, if you want to save the money.
There are now a few other visa options available and supposedly when you stamp-in on visa exempt you get 60 days, so there doesn't seem to be a benefit to the METV, but I am still using it, when I stamp in this weekend I will use my METV and not visa exempt. I am waiting for the dust to clear before I figure out my next visa. (nobody seems to know what is require to get an extension).
In my opinion Pattaya immigration is crooked which explains why extension are so expensive.
My visa Agent is on Pattaya Beach Road, Soi 6. They're in The Victorian (the only respectable establishment on that road).