With a 300l tank you will get through quite a bit of CO2, so I would advise against getting a small system, unless you do as Tim suggests and make sure the reg will work on larger tanks later.
In general it seems the bigger the tank the (much) cheaper the gas. I have a 6.35kg beer gas cylinder, cost £90 to buy, full, and £24 per fill. With a CO2 supermarket reg at about 50 odd quid. I'll bet that works out not much different to what MA would charge for a little JBL system?
I recently got the DD complete system from Aquaessentials for a nano tank. This will take small (500 or 600g) welding gas cylinders which are cheaper and very widely available (I hope, I haven't tried this yet), working v well but would last no time on my 350l tank.