@Bryon-Fortunately, there are some war machines available in plastic. Warlord now do their Roman range in plastic I believe (I think they used to be in resin only-kerching!).
Alternatively, there's the old Zvezda kits, which have been marketed for their old 28mm fantasy wargame range (now sadly out of production) and their own 1/72 range simultaneously!
This means they are a little on the small side, but the Scorpions, IMHO, are still perfectly fine as are the field defenses.The Catapult is also suitable however may be a little later in design than is historically accurate for Romans.
The kits can be picked up for a reasonable price if you shop around, and look very nice, if may be a little basic. I've had some in the past and the Scorpions and siege tower are great, It's marketed for 1/72 and 28mm as mentioned earlier, so openings, doors etc look a little small, but height wise, the kit is fine and a dawn sight cheaper than GW etc.
Crew could easily be made using WF Romans and Nubians and would look a treat.