If you or your company are making seats available to the public for profit you require ATOL coverage for your charter. If it is a 'sole use' ie a company chartering an aircraft as an entity to take its guests on a non-profit making flight then you are OK.
Brokers are generally OK, they do all the leg work for you, some are noticably better than others, but they charge for their services - usually 5% on a charter like this. There is no commitment so you can always ring a few and see what sort of response you get, some of the smaller operations are better than the big glitzy Brokers if you dont want to get hassled too much by follow up calls and broker BS.
You can ring the airlines yourself, phone the switchboard and ask for Commercial, again the response you get will vary from the helpful to the 'what are you bothering us for' to the rip-off, and somewhere in the middle of all that is your answer.
Mainline charter carriers are cheaper than the adhoc specialists if you can find the golden nugget of availability.
If you are looking at a 2 nightstop, it depends on time of year, during the summer, forget a mainline charter carrier as they will have odd slots here and there and not be able to release an aircraft for 2-3 days, the adhoc specialist airlines will, in likelihood, but you will pay through the nose and then some.
You might get lucky with a 2 one way flight on a mainline charter airline and this could well work out cheaper than an adhoc airline on a layover.
If you have a winter enquiry (Nov-Apr) you will have more luck, with availability and your choice of carriers.
If you are looking for a PIK to say MAD with a one nightstop and you found a 733 available, the price could be anything between 30 and 50K depending on how long and when your request is.
hope this helps.