I have been running 6 years now and have only bought a hot box this year. Most jobs can be done cold especially if your just starting. A bare boiler can be bought from as low as £600, I got mine from Brayton Engineering for £1200 all in delivered. A bare boiler needs to be controlled and protected by a relief v/v, you'll need a fuel tank and a trolley if its not being bolted directly into a van. Most boilers are Lavor and are rated at 200 bar (3000psi) @ 15 or 21 ltr/min so if your m/c is rated over this you just have to knock the unloader and /or revs back a bit, to get the temperature right up you would do this anyway. The increased cost over the bare boiler just results in the trolley, tank, cover and the control gear etc that each machine comes with. Do you really need to spend this money at first ?