Planning does expire - I think it's typically after a five year period.
Yes, there's stamp duty on it. There would also be Capital Gains Tax on any profit made.
It would be prudent to have public liability insurance on it, otherwise you are personally exposed. If you buy it, you'll also need to secure it - against trespassers, squatters etc.
I think it's a bad idea to buy, then hold as is. You run the risk of the property losing value, the planning permission expiring and not being renewed (if sough), and its an investment that won't generate any income while you hold it.
Buying it, developing it, then renting it out, or selling it on, makes a bit more sense. Particularly, if its suitable for the long term rental market, and likely to be a sought after location. However, if you go that route, make sure that you've got sufficient funding in place to complete the project - Banks don't like lending on partially completed projects. You'll need a good architect, to oversee the project for you. Also, construction costs are high at the moment, good builders not easily found (and as for them sticking to fixed price contracts, there's always an excuse to have to add to the original contract price, so go in expecting to pay 15% more).
Don't fall for the "it's easy money" line, there's no guarantees here and generally, people coming into property development transactions without relevant experience (and when the market is high, like now) , don't do well with their first time transaction (as what they make, if anything, tends to get gobbled up on paying third party costs etc. when sold only a couple of years later).