As said, there's going to be quite a bit that's not worth the processing time but you'll have to clear it all the same.
You say you've not got any machinery, well not at the moment. So depending on level of access within the wood and proximity to points of final extraction, you could be spending most of your time in man-hauling stuff.
8-10 year cycle for some stuff, depending on market. A few more years for others. Firewood is good of you can get it to customer as ordered on time, in reliable quantity and at profitable cost to yourself. But try to find other markets too. Maybe there's a community pizza oven local to you, that could take the small stuff. If you've got canals close by, the boat owners like 2"-3" dia. small cuts and they're not fussed about straightness. Also, you could try your hand at small scale charcoal making.
Lots of time required to process charcoal but since you'll be getting shot of it anyway... plus, if you get a burn started early, you can be doing other stuff close by for the first few hours. Too much cheap charcoal available at the local DIY stores and petrol stations. You can't compete with those. But maybe you could get your charcoal out as locally produced in a local garden centre.
If there's any straight bits worth milling, that could be worth it. Ok, so it's not going to be an ongoing supply but oak makes great furniture fine or rough. If there's suitable lengths of hazel in any quantity, you could also look into making hurdles. A good use of time while the charcoal is cooking.
So firewood yes. But go after the local niche markets and utilise as much of the wood as possible. If you have a market, you can supply a product. But who's to say you can't make a product and create the market.