First of all, mech is pretty much undefined. The two big mech types/categories that a US player can normally choose from (regardless of period) are: armored rifle and armored recon companies.
My first step would be to decide which type do I want to use. This is mainly because these two types are very different in terms of models that will be used and how the army is handled.
Armored Rifle companies are based around at least 2 big Armored Rifle Platoons. This means that you either upgrade your existing platoons of infantry by adding half-tracks, lmgs, 60mm mortars and tons of bazookas or simply start from scratch and simply purchase a brand new AR platoon (BF has a plastic set now). This type of company is played more like an infantry force so the transition should be smooth.
Armored Recon however, is centered around recce platoons of 1-2 M8 Greyhound cars and recce/60mm mortar jeeps. You might also want to include nuggets like Stuarts or Scott howitzer carriers. Since this is a highly mobile force, able to concentrate and overwhelm a target quickly, you will find it work more like a tank list, meaning your basic infantry forces will not normally be used to build Armored Recon and handling will be completely different. This also means that you will use very few, if any, of your infantry list miniatures to build it.
Second step (and a big decision) for Late War companies is to select whether I am looking for a regular or always attack company.
If I want to go for always attack, I am pretty limited in my choices. The main ones include:
- Armored Rifle from Blood Guts & Glory (with Patton) or Bridge at Remagen (with Mine-exploder platoon)
- Armored Recon from Overlord (both trained and veteran) or again from Bridge at Remagen (with Mine-exploders)
What you have to really be aware of when selecting AA Mech is that you will attack regular tank companies. This means that you need to plan for a good number of effective anti-tank assets (tank destroyers, chaffee tanks, and maybe some artillery). The good thing about it is that you will most probably not defend in missions with mobile reserves, which means you do not need to worry about recruiting any infantry.
Regular list will need to use at least some foot troops to stay effective on the defense with mobile reserves.
After that, I would select the briefing that fits my needs best and finally, put together the list. My favourite one, that follows the advice from one of the previous articles, and the one I will use as an example is 2nd Armored Recon company list from Devil's Charge (1780 pts, LW):
HQ: 2x M8
Platoon 1: M8, recce jeep, mortar jeep
Platoon 2: M8, recce jeep, mortar jeep
Platoon 3: 4x Scott HMC
Platoon 4: 5x Chaffee
Platoon 5: Towed Tank Destroyer Platoon + 4x transport
Platoon 6: Towed Tank Destroyer Platoon
Platoon 7: Full Infantry Platoon
Platoon 8: 4x 105mm Howitzer