Draw up a business plan! I'd start by getting a honest idea of the sale price of any houses, cost to build and a high-level cash-flow. Or why not get the land, with pp, valued - it might be a better option to take the money, pay off your own mortgage and leave someone else to take all the stress?
Theoretically you could make more money than selling to a developer, but unless you have experience in the industry, the risks are huge. The problem is managing the project and that you would be unable to sell until finished and recover your costs if anything went wrong.
Is it outline or full planning permission? Ie do you have any leeway on the spec to make the houses more or less expensive? Have you been given a budget build cost by any architects or builders? You would normally need quite a bit of cash up front as banks normally want to see progress and then lend on work done. Ask the Eco Building Society as they help self builders. Could you manage to phase it like the big developers - ie sell one off plan then use cash to finish off the builds (risky). Or build one and then sell to build the others? I have ever found MN to be great for advice on money/investments so probably best to find some specialist property forums to ask. Good luck