If you're carrying foot infantry into a fight your first priority is getting them into cover. That means you should ideally be able to unload them straight into cover, which means you want a tracked chassis than can actually get in there without getting stuck.
And you want a cheap carrier, which means you should keep it small - which in turn means speed, not armor, will be the main defense and armament will be limited or nonexistent.
RaceTrack light APC
Mass: 18 tons
Tech Base: Inner Sphere
Motive Type: Tracked
Rules Level: Experimental Tech
Era: Clan Invasion
Tech Rating/Era Availability: E/X-X-E
Production Year: 3070
Cost: 404 734 C-Bills
Battle Value: 304
Power Plant: 145 Fuel Cell Engine
Cruise Speed: 86,4 km/h
Flanking Speed: 129,6 km/h
Armor: Heavy Ferro-Fibrous
Armament:
None
Manufacturer:
Primary Factory:
Communications System:
Targeting and Tracking System:
================================================================================
Equipment Type Rating Mass
--------------------------------------------------------------------------------
Internal Structure: Standard 8 points 2,00
Engine: Fuel-Cell Engine 145 6,00
Cruise MP: 8
Flank MP: 12
Heat Sinks: Single Heat Sink 1 0,00
Control Equipment: 1,00
Lift Equipment: 0,00
Armor: Heavy Ferro-Fibrous AV - 69 3,50
Armor
Factor
Front 23
Left/Right 16/16
Rear 14
================================================================================
Equipment Location Heat Spaces Mass
--------------------------------------------------------------------------------
Anti-Missile System FR 1 1 0,50
Infantry Compartment (4.0 tons) BD 0 1 4,00
@Anti-Missile System (12) BD - 0 1,00
BattleForce Statistics
MV S (+0) M (+2) L (+4) E (+6) Wt. Ov Armor: 2 Points: 3
8 0 0 0 0 1 0 Structure: 0
Special Abilities: AMS, ENE, EE
The RaceTrack doesn't carry any offensive weapons, its armor is limited to taking a single serious hit on any side, and it carries both an explosive power plant and ammo without CASE.
But it can hit a +4 TMM over broken terrain, carries an AMS for extra protection, and costs only ~400,000 C-bills. The explosiveness is more of a theoretical than a real disadvantage; you're far more likely to die from straight-up damage than a TAC...
Like Gomiville's design it's a pure transport. Run at flank speed until you get to the fighting position, then slow down to drive into it and drop your droops.
*********************
For a different take:
M299 support IFV
Mass: 29 tons
Tech Base: Inner Sphere
Motive Type: Tracked
Rules Level: Introductory
Era: Clan Invasion
Tech Rating/Era Availability: D/X-X-C
Production Year: 0
Cost: 410 658 C-Bills
Battle Value: 462
Power Plant: 145 I.C.E.
Cruise Speed: 54,0 km/h
Flanking Speed: 86,4 km/h
Armor: Standard Armor
Armament:
1 LRM-5
1 Infantry Compartment (4.0 tons)
Manufacturer:
Primary Factory:
Communications System:
Targeting and Tracking System:
================================================================================
Equipment Type Rating Mass
--------------------------------------------------------------------------------
Internal Structure: Standard 12 points 3,00
Engine: I.C.E. Engine 145 10,00
Cruise MP: 5
Flank MP: 8
Heat Sinks: Single Heat Sink 0 0,00
Control Equipment: 1,50
Lift Equipment: 0,00
Armor: Standard Armor AV - 120 7,50
Armor
Factor
Front 40
Left/Right 30/30
Rear 20
================================================================================
Equipment Location Heat Spaces Mass
--------------------------------------------------------------------------------
LRM-5 FR 2 1 2,00
Infantry Compartment (4.0 tons) BD 0 1 4,00
@LRM-5 (24) BD - 0 1,00
BattleForce Statistics
MV S (+0) M (+2) L (+4) E (+6) Wt. Ov Armor: 4 Points: 5
5 1 1 1 0 1 0 Structure: 1
Special Abilities: EE
The M299 isn't the best design possible, but it's about as simple to build and maintain as you can get. All SW-era tech. Still able to hit a +3 TMM on the straightways and take a pair of heavy hits on the front or sides it can also support its platoon at long range with its LRM5. I'd suggest loading half the M299s with smoke rounds to cover the advance while the rest packs HE to keep the enemy honest if they stay out of infantry weapon range.