You may want to run the serial number through a welding supply store to find the age of the machine..might be pretty old...doesn't mean bad, just may be hard to find parts for and everything rots over time so insulation on various things may be on the edge.
You need an AC/DC unit to run aluminum...which is usually the case with transformer machines since they are inherently AC and have rectifiers to convert to DC. The HF is just an overlay current used to start or stabilize an AC arc. The HF switch is set to start for DC work and continous for AC work (unless doing stick and the HF is not needed).
The advantage of old machines is the price...usually pretty good. Check
www.ebay.com and search the completed listings (advanced search) to get an idea of what people pay for these things. Getting whole rig that works up front will not blindside you like getting a new machine and finding out that all the other bits and pieces cost a small fortune.
Disadvantages compared to new:
AC is sinoidal. New machines are square wave AC. The sinoidal wave needs the HF overlay to keep the arc going during the wave swing past zero. Square wave is so quick on the switch that the arc is inherently stable.
Transformer machines are heavy (had an Air Products clone of a Dial Arc 250 that was
475#). The big transformers waste power heating the copper wire and iron core. Big fan is noisy. Power requirements are huge, especially on single phase...but you rarely use that sort of output.
A new inverter machine weighs 1/10th. Power requirements is about 1/2 (they don't heat up like a transformer, more efficient). Square wave. Lots of whistles and bells in adjustment, pulse, etc. Big price tag, all the accessories to buy.
Sorry for rambling, hope some helps.