I'm just getting into the coilgun game myself, I'm planning to use less power than you are, and I'm FAR from any sort of expert here, so please consult people other than just me, but I have some electronics knowledge and I think you're on the right track. I'm planning to use MOSFETs too, but keep a few things in mind:
1--MOSFETs have an "on" resistance, which can be found on their datasheet. The lower the on resistance, the better. Generally speaking, higher voltage, lower current MOSFETs have higher resistances, while lower voltage/higher current MOSFETs have lower resistances. I found what may be a good one for what you are doing, but, well, it ain't cheap:
TK100L60W,VQ on Digikey it runs $31.20, each BUT it's rated for 600V, 100A with an astounding on resistance (by MOSFET standards in this voltage range) of 0.018 Ohms! Bear in mind you don't have to use this, and depending on your capacitor's uF (microfarads) and your coil resistance, you may not need a resistance this low.
You can put them in parallel to lower the resistance, but if you do, leave some margin of error because the distribution won't be completely even. Bear in mind that during the brief time you are discharging that cap, you are creating some intense heat: Power dissipated (Pd) = I^2*R so Pd = 100amps^2*.018ohms = 180 W In fact, given you are intending to run 100A of current through it, I would recommend using at least two of the ones I mentioned.
2--Mind your gate voltage and, importantly, reach your target voltage quickly. MOSFET gates are like tiny capacitors and must be charged before the MOSFET is completely in the on state. Between on and off, it is acting like a resistor which means during that time it is soaking up power from your circuit by turning it partially on and, worse, generating more heat, which can damage it. Don't let this scare you off. Simply make sure you are charging it with a reasonably large amount of current (an amp or more, depending on how fast you want it to charge) and you'll probably be fine. By turning on the MOSFET quickly, you can significantly reduce the amount of heat it generates and ensure that more of the cap's energy goes to your coil. Don't forget a bleed resistor to ensure the gate discharges after the power to it is turned off.
3--Keep in mind the total resistance of your circuit relative to the on resistance of your MOSFET. Your circuit's resistance is the total of the following, IF everything is in series:
B--Capacitor ESR (Equivalent Series Resistance, look for it on the cap's datasheet)
C--MOSFET on resistance
D--Coil Resistance--If you don't have the means to measure it, estimate the footage of wire used to make it and multiply that by the resistance per foot of wire for the gauge of the wire you used. There are plenty of charts/calculators for this on the web.
If you notice that total on resistance of your MOSFET(s) is a large part of the total resistance, you may want to rethink your design a bit. Basically, whatever percentage of the total resistance is coming from your MOSFETs is going to be the percentage of the cap's power that they'll be dissipating. That's less power for blowing holes in things! If your circuit resistance is too low for your MOSFET you can make your coil bigger to increase its resistance and the extra number of turns should make up for the lost current, within reason. The bad part about increasing coil resistance is you decrease overall circuit current (although a larger percentage of what's left is now going to your coil) and increasing your cap discharge time, which may or may not be bad depending on your present discharge time. You can add more MOSFETs in parallel to lower their total resistance, but if you then make sure they're all still getting sufficient gate current when turning them on.
I hope this helps you out some. Sorry to ramble on, it's late here and I'm only semi-conscious at this point :P Best of luck, let us know how it works out!