You are correct - Once that metro line is up and running (suposedly sometime around September this year, but could be later) properties along the line (between Syndenham and Bankstown) will be more in demand. That said, the entire corridor has been rezoned around the metro stations, so there will be a lot more new supply coming to market over the next few years. So buying an older unit may seem ok, but you could end up being crowded out at a later date by flashier new builds.
Campsie is certainly under-developed, but to the north end of Campsie there are already a lot of 2 storey walk-ups that are unlikely to be fodder for the developers. Around the station (on Beamish and Campsie Streets) there are already signs of new developments, and I think the developments will focus on the area to the south of the station (between the station and Canterbury Hospital), thus units to the north (between the station and the Cooks River) won't be overly impacted by the new supply. Units along Caterbury Road are generally a "no go" in my opinion, as they are a decent walk from the station, noisier, and seem to be lower build quality. So for Campsie, stick to the 2nd, 3rd, 4th, 5th Avenue areas, closer to the Cook River but still within walking distance to the station.
Belmore is similar, but in my opinion less likely to see as much capital growth. The demographics are vastly different, and the economics (the shops and restaurants) not the same as Campsie. If asked to pick between the two, I would certainly opt for Campsie.
All that said, in that metro corridor I don't think either of these are the best. I would suggest looking at Canterbury (some of the Charles Street developments) and perhaps even look at some of the upcoming developments (there is a three block development on Canterbury Road that just got it's DA). There are some older, two storey walk-ups in the area as well, further away from the station but only a gentle walk to station and shops. Alternatively, look at Dulwich Hill or Hurstone Park. There are some nice older blocks around Hurlstone Park which probably fit your budget. For example, 6/56 Crinan Street (1 bed, 1 bath, 1 parking) just sold for $560k, with two-bedders going for $650-$750k. A superb little pocket, with scattered re-zoning. In my view, much more likely to see capital gain, far more rentable, and a much better investment than Campsie.
The general rule of that area is "don't cross the creek". That is, once you are south of the Cooks River your demographic changes, and the economics change. North of the River (Canterbury north, H/Park, Dulwich Hill, Marrickville, etc.) and you are in nicer, more well-established neighbourhoods.
And south of the River - Campsie is ok, Belmore a solid no (from me, at least), and Belfield is ok (although lacks transport).
