Over the holidays I have been looking at available miniatures.
Probably the best I have found that covers the vehicles seen in Twilight is Battlefront's TEAM YANKEE (named after the book). These are 15mm or 1:100 (although 15mm can be 1:120) scale.
While they are expensive the newer models (there are still some older kits that mixed resin with white metal) allow you to build different variants of the same base vehicle ( the M113 can be M113, ACAV, M106, M163 VADS or M901 ITV for example) and you get addition parts that can be used to customise other vehicles.
You're looking at around 8 USD a vehicle, and you need to assemble and paint.
They also have infantry that aren't the greatest on detail but at the scale are adequate plus they include heavy weapons.
Long time ago I use to play a game called Tractics using 1:72 scale vehicles and soldiers. You can do terrain really cheaply - I design a building on Sketchup, print on A4, glue onto old carboard, cut and assemble and you have a workable building for 50 cents. Get a large piece of green material (tablecloths are good), and use newspaper under the cloth to form terrain. Any old material can be cut up to make roads or waterways. Rather than use premade trees buy a big bag of dyed moss and collect sticks to make trees.
Works fine and as a Twilight battle isn't going to involve huge numbers of vehicles you can do it for maybe a few hundred dollars at most. Then use rulers or tape measures for movement and range converted to scale.
In Australia you can buy a large folding table from Bunnings for $35 dollars. Put two together and you have a big enough area for a battle.
For most purposes battlemaps and counters are fine, but if you want to do something special there are options.