You might want to do some market research as you might find that there really isn't much of a market out there at the moment.
Existing agents are struggling, getting rid of staff and offices so it's not really the most promising idea for a new venture. Not at the current time anyway.
If it it what you want to do then why not look in to taking a franchise as there are companies out there who do this.
You buy in and get to use their name etc...
If you wanted a better business plan then go into lettings then pick up on sales when the market is better.