They sound good when running well. A bit faster than a BSA C15. Only holds about 2 pints of oil in a tank which is inside the crankcase. The gearbox is separate although it looks like unit construction. The primary chain adjusts by rotating the gearbox.
I had a Matchless G5 version with decompressor.
It will cost more than its value to restore but most bikes are. I have looked at a few G5s in the last 10 years but all were in very bad shape.
Find a really nice 250 version and it would be worth buying at 2500. They are not very lightweight though. Quite a large bike and bigger than the BSA C15 in size.
The bike in the picture might be worth more for the number plate.