Have a look at this website: www.commutefrom.com - tells you which towns are on which train line into London.
Having just had a look at Right Move (which lists practically all rentals and houses for sale in England), Watford would be affordable, just about. You are right that St Albans might be pushing it, but as Watford is a short drive from St. Albans, you can live in Watford, pay far less to rent, and easily get to St Albans for shopping, restaurants and the nice park and Cathedral! And Watford is only 20 mins from Euston on the train,.
Also, just to echo everyone saying that you would be v lucky to fit five people in a 2 bed flat - even if your children are small, you will really really struggle even to fit a double bed, a toddler bed, a cot and a single bed in a 2 bed flat here. In fact, given the size of 99% of rooms in flats here, I would say it's almost impossible and suggest you look at 3 bed houses (which migh put not necessarily be massively more expensive - eg a 2 bed new flat 5 mins walk from the train station could be the same price as a 3 bed house 15 mins walk away).
On your question about getting a real estate company (called here "estate agents") to search for you - really not recommended here for most normal people. There are loads and loads of estate agents marketing properties in any town, and if you just rely on one to search for you, they will just be telling you about houses on their books and you will miss the other 75% of the market which they don't cover.
I think you may be able to pay someone if you are relocating from abroad to look for houses for you, but they won't be an estate agent as such. Might need to ask on an expat forum for how to find that kind of person / company?