In terms of hidden costs, you need to consider :
public liability insurance (less than £150 pa, unless you are buying a forest rather than a wood)
employee liability insurance (if you are going to have 'helpers', paid or unpaid, in the wood. The definition of an employee seems to be anyone acting under your direction - approx a further £300)
general maintenance ( a boundary with a field may need a fence to keep the cows / sheep out of your wood - fencing may cost only £4 per metre, but a field boundary can be many metres. A wind-blown tree which is resting on another tree or trees can be difficult and dangerous to fell - a tree surgeon may be an expensive luxury - some charge £300 per day or more)
pests (getting rid of pesky rabbits or squirrels may be free if you know a man with a gun, but deer can cost more in terms of fencing to keep out)
safety inspections (you need to evidence that you have given due diligence to the needs of woodland visitors, be they walking a public right of way through your woods, or trespassing - could cost a few hundred pounds per year, depending on the size of the woodland. You will need someone qualified and insured to evidence the inspection - your insurers may not pay up if you have ignored obvious dangers)
obvious costs
tools - chainsaws, together with all the safety gear, loppers, bowsaws, axe etc
travel to and from the wood on a regular basis
income
depends on what you have, and whether there is any protection on the trees, (mainly, but not exclusively, Tree Protection Orders, and SSSI designation). In any case, any income you can generate will be modest - more like pocket money than a proper job! If you want to fell more than 5 cubic metres in a calendar quarter, you will need a felling licence from the Forestry Commission - that's about one mature tree
grants
the main source of grant income is from the Forestry Commission Woodland Improvement Grant. If you intend to open your woods for the public benefit, grants can run at 80% of costs, but if you want to keep it private you are limited to biodiversity, which except for a few areas of the country is limited to 50%
If the woodland is considered to be an asset, (perhaps based on ASNW status, or SSSI), then grants can be quite generous. I am getting a WIG on 5 acres of woodland for clearing rhododendron, thinning sycamore, clearing some 'glades', replacing a stock proof fence, replanting with native stock etc - I have to 'spend' approx £10,000 to get £5,000 back in grants, but I can do quite a lot of the work myself, and have found a guy who can do much of the work at less than the FC 'standard costs'. I will have to spend some money, but at the moment I believe that my costs will be neutral at the end of the 5 year period of the grant. Again, depending where you are in the country, your WIG, or parts of it, could be based on the new 'woodland birds' programme, which is funded at 80%
summary
buy it, and enjoy it. Based on my experience, it is the best investment I have ever made, certainly in terms of the feel good factor. I suspect that many people here will agree.