I guess it depends on the undersuit. Personally, I prefer the moisture wicking under armour, or some other varient (starter, nike, etc...) . It really comes in handy when it's hot, and you REALLY dont want anything else than the armor you're already weraing to hold in that heat (and that black armor is an oven). Another thing I've noticed over time is you want the tight fitting (compression) gear, maybe a size larger than what you may normally wear, or the loose fit stuff, but a size or two smaller than what you would normally wear. The reason why is I hate having any loose material flapping out of the gaps in the armor (elbows, arm pits, knees, hips, etc..) It helps when slipping in and out of the armor and in general just looks better, not to mention helps you deal with the heat.
Another thing to keep in mind, when adhering velcro to your armor, always have the loop (soft/furry side) of the velcro facing in towards you, while having the hook (hard/plastic side) facing away from you. The reason for this is that hook will grab a hold of your undersuit and will ultimately cause damage. BTW, I'm not saying to put velcro on your undersuit, I'm refering to the velcro you put on the armor, where velcro is necessary to hold pieces together.
Just throwing those things out at you.. but I suppose the first thing is to get what you can afford, then upgrade to the better gear when you can afford it.