pscan.uk

pscan will work fine on old laptops.
If you can manage it then I would recommend a lightweight Linux such as xubuntu.
Personally I use an old Lenovo T420s, and I also use old Dell D620 and D630s, but just avoid the ones with Nvidia graphics (the ones with Intel graphics are fine).
Mucking about in the garage is a kind of harsh environment; it is so easy to trip over a cable or drop it on the floor, and I think a dedicated cheap old laptop is the way to go.