I think an awful lot depends on what boats cost in Tokyo. If you can buy a nice Albin Vega for $6-7K then this one would be pretty steep if FREE.
If that trailer is decent and IF you have a place to work on it once you tow it there there may be something. If you have to drive to work on it each time forget it.
If you are really meticulous with your work and love restoring junk to something decent then maybe. If you are not meticulous you may end up with painted junk that you have wasted $5000 or more on and still is worth ZERO.
You need to determine first if the hull problem is cosmetic or if it has compromised the structure of the hull. Gelcoat protects the hull from water and UV and in this boat that may not be the case. Hire a surveyor and go from there.
Assume the engine is toast and if you can make it work then that is a bonus. A 10HP outboard is $3000 new and even an old one working well is $1000 - and you would need a bracket, etc... that costs $$$
Things to check. Hull integrity, rigging, mast, etc.... You will need all new halyards and lines as the ones left out for 10 years will likely be rotted. Sails you will have to buy from a used sail swap place - determine the measurements and then start looking - I,J, E, P are the numbers to look up on Mauri Pro or US Sailing PHRF site. New sails are more expensive than the boat is worth.
Inform the current owner that you love boats. That you think of the Albin Vega as a classic and would like to see it restored to the condition it deserves. Let the owner know the boat in excellent condition is worth less than 10000 and that average condition is 6-7000. Let him know that you expect to require replacemnt/restore of engine, rig, hull, deck, cushions, etc... and replacement of sails. Let him know that you have researched these costs and thy exceed $7000.
After all of this if the owner does not give you the boat forr free then walk away. The only two initial costs you should pay are for a survey to determine if the hull is even worth saving and transport to put the boat near you so you can work on it.
Look up Glissando a Pearson Triton 28. There is an extensive website on the restoration of that from a wreck into a stunning classic boat. Make no mistake that this Vega is a wreck. There is nothing really redeeming about it except perhaps the trailer - and looking at the condition of the boat I have serious doubts about the trailer.
I am not trying to be negative. It is possible to restore a boat in this condition. It will take YEARS between two and five depending on how much time you devote. The materials are not cheap either so be warned.
Good luck with your project should you take it on. Consider posting progress and pictures on a web site.
and most importantly remember one key thing - YOU WILL NOT SAVE MONEY BUYING THIS BOAT. By spending LABOUR (your own) and working on it over a year or two you are buying a boat over time but still paying at least as much and maybe more.
It can be worth it but you certainly should be aware what you are getting into.