Paper is very inexpensive and if taken care of can last a loooong time. I'm still using the same roll of white paper that I purchased back in 2008. I have no idea how many shoots its survived through but I think I'm safe to say at least 60-80. I'm just getting to the point where i've had to trim off enough paper that for a model any taller than 5'6 I do not have enough paper to roll out. I'd say that for 4 years and that many shoots, the 60$ was well worth the investment.
Regarding plexi for the floor, yes any home hardware store should sell some plexi for your needs. Ideally a 4'x8' sheet would be used as it not only gives you alot of reflection in a wider shot, but also covers much of the paper allowing it to be saved from damage. The drawback to this size of sheets is the sheer weight and cost.
If you can find a 3'x4' or something close to that size, go with the medium thickness, roughly 5/16 or a little thicker and you should be fine. The danger with going too thin is that a girl with some serious stilettos can easily crack the plexi.
If you are to purchase monolights, I'd probably recommend going with the speedo monos, I forget what they called the line... I say this because the color temp of the bulbs and flashtubes should match that of the pack based system. If you purchase a different brand of mono you're likely to run into issues with blending the color temps of the two different brands of lights.
If you like the pack based system, just scoop up another pack and another couple of heads.
I do NOT recommend trying to run all 4 lights off of the one pack. It is doable, but trust me, its a huge pain.
Even with splitting the channels on the pack to differentiate which heads get more power, I can 99% promise you that you'll be put into a situation where you can't adjust the power enough + or - to get where you need to be and then you're forced to move your heads back or forward, which depending on your space and limitations with the head cables, may not even be possible.
If you're able to at the minimum split your background heads and the subject heads, you'll have many less headaches.
just my .02c