>>1732
Arboriculturist here. Think I can help and give you some advice and confidence OP. Your plan is similar to mine, once I've obtained some land.
>Solid boards sells at about 250€/m^3, but I'm no farmer, I have no contacts, let's use 100€/m^3 as best case scenario, and 20€/m^3 (firewood) as worst case scenario
Firstly don't forget you'd have 8 years to make contacts while they grow. I don't know if those figures are for generic "timber" or this specific wood, but I think you could expect to sell it for near or full market value, especially if you export it to Japan.
>500 trees cost about 3750€
>I do not own tractor, so planting more than 1 ha is impossible.
Grow them from seed. Plant 2-3 seeds per tree you want and thin them out once they've germinated. Seed-grown trees are much more adapted to their environment than imported standards, therefore they grow faster (often overtaking immigrant trees within 5 years) and are less susceptible to local bad weather, diseases and pests (expect to lose 2-5% of your crop to these). Seeds are also about 1000x cheaper to do and you could plant all 20 hectares yourself by hand over a few weeks. The standard trees you would otherwise buy are probably no more than 1-2 years old at most, these things grow at 7 feet per year.
>calculation price of tech and fuel
I don't think you would need any tech or fuel before actually cropping them, aside from deer fencing. Your young trees will be a magnet for rabbits and deer, you need to fence them out and shoot the ones that will inevitably get through. DO NOT use chemical fertilisers or fungicide/pesticides. You will kill your mycorrhizzal fungi network and soil food web.
On that note, other advice I can give you:
>Mix planting, cultivate healthy soil food web
I would make maybe 30% of the tree crop mixed fruit trees/berry bushes spread evenly throughout the land, and also including a shelter belt of oaks, ash, hawthorn and beech on the north/eastern side will be worth it to increase your biodiversity significantly and filter cold winds. Your biggest problem will be Pawlounia-liking predatory fungus and pests, but having a few different species in the mix will help reduce their chances of dominance. Pawlounia are N-fixing, so you will have insanely huge fruit crops which you could just use yourself or collect and sell to cider brewers/local farm shops.
>Invest in the soil fertility
I can't stress this enough, do not use chemicals. They kill mycorrhizal fungi and soil bacteria. Mycorrhizal fungi are worth researching if you haven't heard of them (I recommend "Teaming with Microbes", Lowenfels and Lewis), but basically they are symbiotic fungi that increase the effective rooting area of a tree by 900-1000x (pic related). You can buy spores and sprinkle them in seed planting holes throughout the plantation; in a short time after germination they will link and "infect" every tree, making the entire area one symbiotic web and stopping the predatory ones from getting a foothold. Soil bacteria are the actual agents of nutrient uptake by roots. It's probably the biggest mistake farmers make nowadays; the more chemicals you put on your land, the more you kill it, the more chemicals you need to artificially sustain the life that you want to grow. It's a vicious and expensive cycle. Artificial fertilizer is abhorrently inefficient; more than 90% of it misses the rootzone entirely and leaches straight down to the water table. I would invest in some mushroom compost to begin with just to kickstart your soil food web and get some earthworms working away in there. After that you might not need to even fertilize again (fungi can transport trees nutrients from within a radius of 40 miles) but if you do, do it with organic compost like manure or mushroom.
Good luck, let me know how it goes, if you do it/don't do it and why. I will be doing pretty much this when I have my own land.