A multi-million (billion?) pound facility in a rough area of Manchester??
Seriously though, if as blackwatch says it is a bad district - it ain't worth it! If you want to see 57's, take a day trip to North Wales or Holyhead - 2 run arounds a day Monday - Friday!
Longsight is hardly a multi-billion pound facility!! Its a bit of a archaic dump, that (other than the Eurostar depot & the refurbed Pendo bit) hasn't really changed much since it was a diesel / electric depot from the 1955 modernisation plan (if not before in kettle days!)
I think maybe you are referring to Ardwick depot, which is possibly a mult-million £ brand new depot (where 185's live), it is on the site of the old carriage sidings.
Longsight / Levenshulme isn't a nice area at all. It is full of "yardie" style black gangs who I imagine if they saw a weak-looking "trainspotter" perhaps with a fancy camera and a notebook, they would think of as easy pickings. If your desparate to go, a car would be useful perhaps (as at least you can then stop, do what you want to do, and get off pdq) other than that - I wouldn't risk it. Ardwick depot is the same, as it is roughly in the same dodgy-type of area, although it does have the added bonus of being near the "prossy" area of Manchester, so if your prepared to pay for a bit of nooky as well as some unit numbers........