If you can find an old miller synchrowave 250/250 that would be my first choice.
Its an older machine but is built tough. You can run straight and reverse DC polarity, AC and it has HF as well and you can stick weld with it also.
I dont see many come up for sale here in NZ as they are popular and very reliable.
More modern machines can be pricey and these days they are mostly inverter type power sources.
Personally I prefer a chunky transfomer and a good bridge, usually you get a much nicer arc and you dont need to be a rocket scientist to operate it.
On aluminium you need to run continuous HF on AC with pure Argon gas. An engine block would probably need to be pre-heated as well to help the weld "wet in" to the parent material.
The Lincoln machines were also very good up until a couple of years ago ( in NZ at least ) when they changed thier diode supplier and all sorts of wierd things started to happen. They may have come right now but I havent had one in to check out for a while.
If I were you I would still try and find a GOOD machine shop, doing it yourself could do some serious damage if you are a novice Ali welder.
Try and find a place that does boilermaking or high pressure pipe welding, usually they are pretty good tradesmen and tend to enjoy a job that is a bit out of the norm for them.
If you want a home / hobby machine then mostly they are scratch start, dont get one of these. They will be single phase and I dont think you will get the heat output out of it that you need to keep the work piece hot enough to get a good weld.
Make sure if you buy one it has HF.
If you are welding Ali be very careful as there are very severe radiated arcs. Completely cover you arms with a dust coat and wear leather gloves and a welding helmet of course.
If you dont you will get badly burnt.
Shop about, stay with a known brand for good after sales service and be prepared for it to be a bit costly if buying new.
Cheers
Wayne