Just off the top of my head I would say, make it random.
The 4TP (400xp) Custom IS Vehicle is a Random Stock Vehicle (native I.S. affiliation)
actually that's not correct.
CUSTOM VEHICLE TRAIT TABLE
Trait TP Inner Sphere Vehicle Selection Clan Vehicle Selection
0 Random Stock (own I.S. Affiliation) N/A
+1 Random Stock ( any I.S. Affiliation) N/A
+2 Choice of Stock (own I.S. Affiliation) N/A
+3 Choice of Stock (any I.S. Affiliation) Random Second-Line Stock
+4 Design own Vehicle (I.S. Technology) Random Front-Line Stock
+5 N/A Choice of Stock (any Clan)
+6 N/A Design own Vehicle (Clan Technology)
as I see it the 4+ means I can design a vehicle from SCRATCH for the weight class I can have.
an additional note is :
A Clan player with no Custom Vehicle Trait will receive a Random Front-
Line unit assignment if trueborn, or Second-Line if freeborn.
bear in mind, I would add a few "crevats" to this but in general the custom table is intended for innersphere chars.
so you need (in 3075ish) 3 points in custom vehicle to even get a clan mech, if you aren't of a clan faction.
also I personally would allow some "reasonable" customizations for ~1 tp equivalent, (not including the 4+ point level)
example of what I consider "reasonable would be something like taking a PXH-3M phoenix hawk, and dumping 1/2 of the mg ammo, for a second machine gun for example