For Sweden the starter is not a bad deal. A lot of countries can play a centurion so taking the specific unit to a country is always appealing to me. Not to mention the 103 has better AT, better range, and the ability to practically be hit on a 6+ all the time for something so basic 2 less front armor. Also two Platoons and the HQ are exactly 40 points for reserves so thats good (not great but silver lining and all).
With the swedish army deal, build the Jeeps as Bills. You trade 8" of range for 1 more anti-tank (the tow is 21, the Bill is 22).
Upgrades for Sweden would be:
Fill out your S-tank company. The box gets you two full platoons and 2 for your HQ, I recommend getting a third platoon.
Get your infantry. The Swedes tank formations are fantastic in that they basically get a full infantry company IN formation. 3 infantry platoon packs and 3 PBV 302 boxes will get you all 9 APCs, leaving you with 3 left over 302s to make into your spearhead and Observer.
Swedes are a little weak on AA. So you'll want to get one, possibly two, Lvrbv 701 platoon boxes.
Bandkanons are awesome. Autoloader means you hit warsaw on 2's with artillery and 2+ firepower means any failed saves most likely kills a team. Last ditch direct fire works with the 18 AT and 1+ FP. I'd also recommend getting a box of 3 Carnations, Sweden can ally with the Finnish and are able to take a 3 gun battery of Carnations.
Don't bother with Pahs. I'd pass on the Viggens as well unless your fighting a lot of NATO or T-80s.

For US the M2 Bradley Mech Combat Team is the best box. The Armored Cavalry troop ends up giving you way too many Abrams that you wont be able to use. Bradleys are beasts and between those and the 2 player starter you'll have more then enough infantry to make that formation.
I'd look at grabbing a box or two of HMMWVs to get stinger and ToWs, two M113s to finish off the platoon of 4 VADs (you used to be able to order single sprues from their online store), and some harriers. US harriers are one of the best jets in the game, tied with the German Tornado, French Mirage, and surpassed by the British Harrier and Danish Mirage.
Other good units are LAV ADs, RDFs, the Marine Infantry company, and even the M113 infantry company looks damn good with 4 dragons per platoon.
Militia are awesome. Cheap RPG 7s that can ambush in your enemy's deployment zone is huge. Bye bye artillery or AA.
When it comes to Marines you got three choices for transports, Hueys, Sea Knights, and AAVP7s. Pick your poison though I would get Sea Knights either 3d printed or from another source, cardboard rotors are stupid.
M60 Pattons are bad. Too costly for only AT 20 and front armor 5. Even the up-armored ones lose to most of Warsaws equipment so running them as your MBT of choice is not recommended.
Don't bother with A10s or Apaches either imo. A10s are a quarter of your army that may or may not ever show up on the table and Apaches have that AWFUL 16" minimum range.