Yep, you should get plenty of fills from a six litre 300 bar bottle. I can't tell from your photo what type of valves they have but I'm fairly sure that it's a DIN 300 (G5/8). If so, you will need a DIN 300 fill whip. There are plenty of different vendors on eBay, AliExpress or Amazon that carry them and they are relatively cheap. I would still recommend getting a hydro especially if the bottle is supplied unpressurised. You have no idea of the tanks previous life or how it has been stored since it was decommissioned. If it contains rust it may still be OK. The hydro testers will make the decision whether to have it rumbled and passed or if it has deep pitting they will condemn it and drill a hole or cut it in half. I nearly bought one of these when I was looking for my first tank. I didn't think anybody else would be interested so I only put a lowball bid and missed it.