As you're probably aware, the miniatures aren't being produced any more so you'll be limited to what you can pick up privately or scavenge from remainders in stores. Could be an issue for putting together an Arachnid force although you can find decent proxies for Mobile Infantry and Skinnies if needed (if you're doing both sides from scratch it might be worth considering looking at some of the 15mm stuff out there that could do as stand-ins unless you're dead set on having the movie/CG series look).
Don't still have the books , but I came across my campaign files the other day so I can ballpark it until someone with the rulebook pops by. There's loads of options for each force, so this is purely my take on a standard set of forces (added in the Skinnies too, just in case).
I'd suggest 2 squads of MI as a decent-sized basic force (1-2 command figures, 10-16 troopers including 4 heavy weapons, perhaps a Marauder suit replacing one heavy weapon and a couple of troopers per squad if you can get them). IIRC 2 squads is the minimum legal force for MI. Skinnies would need about double the squads to match that. Arachnids 40+ Warriors (including other common flyers/ranged combat types), two Tankers and a Brain Bug (start it underground and you don't need a model
). That's about 1500-2000pts a side depending on options and extras taken, which IMO is the game's "sweet spot".
The Army Books - if you can find them - add a number of other options (notably the extra-heavy Heinlein-ish powered armour suits in the MI book, only need a half-dozen or so to match the above force, the heavier/specialist Skinnie troops and other options in their book and the movie-style light infantry in the Klendathu book which are about half the points value of the powered troops).
