This is an old system with many flaws. I've documented "house rules" before, but can't seem to find them. So I'll post them here quick. For the record, I'd recommend you ditch d20 Modern for True20--all the same benefits and fewer flaws. However, I'd take the Dramatic Chase rules from Spycraft.
#1: Dump cross-class skills
Characters get to spend skill points on any skill that any one of their classes knows 1 for 1.
#2: Wealth Check multiples
Every doubling of quantity adds +2 to the wealth check Purchase Check.
#3: Wealth and Cash
Sell the suitcase of money as if it were an item to determine if the cash amount has any impact on the character's wealth score. Alternatively, let the players keep the cash for other purchases.