You don't need an agent to apply for the Elite visa. You head to the thailandelite. com (remove the space before "com") website, start the visa process, and then wait for the approval. Elite says it can take about 3 months, but on an average it seems applications get approved in 4-6 weeks.
In case you really want to use an agent, again, head to that website to find the list of official Elite agents. The agents are not supposed to charge you an extra fee; you only pay the official Elite fee.
With the Elite visa, the money that you pay is gone. With the retirement visa, you get to keep money in your bank account.
With an Elite visa, you do get to live in the country for years. However, you still need to do 90 day reporting and yearly-extensions, like other visas. Doing both these is super easy though, and the Elite team helps you do all of this.
You can get a retirement visa if you're eligible for it. In my opinion, though, Elite is the easiest visa to get (of course, if you're willing to shell out bucks for it).
There are multiple memberships. You can get the visa for 5 years, 10 years, or 20 years. The 5 year one costs a one time upfront payment of 600,000 THB. Also, if you consider applying, always apply from their official website, which is thailandelite .com (remove the space before the dot).