it should fit, micro atx is just like a atx board but in a smaller form factor, it should fit, it just wont use all the slots at the bottom of the case cause the mobo is'nt tall enough or have as many pci/pci-e/isa slots.
for the back pannel usually a mother board will come with a I/O shield plate, you can get standard shield plates but a lot of boards are puttings usb connectors everywhere so they've become somewhat custom..
honestly i'd see if the mother boards mounting plate can be taken out of the case.
probably not since most dells i've seen are fixed.
in which case fire up your rotary tool and cut it out.
there is litttle to be gained with a case inside of a caibnet it will surely lead to higher tempatures, take up more room, add to weight.. only thing it might be useful for is if you have loose quarters running around or if by some freak accident the monitor collapses on it.
if you dont want to cut on the case make your self some stand offs.
number of ways you can do this.
you can use some plastic or rubber washers..
or you can make some out of a ball point pen.. just take it apart cut it up.
they dont even need to be exactly the same size although the closer they are the better.
then lay them on a piece of wood, line them up with the mounting holes on the board, then put a small would screw, 3/4 inch would do the trick.
some people will run a ground wire to the psu from the mobo, there is nothing wrong with this as in a metalic case they would be connected though it's mountings, but it's always been my understanding this is more out of safety then electrical necessity, if there was a fault the case would be grounded, without the case being grounded it could possibly shock you, but no case no possibility of shock
with that said you dont have to if you dont want to, there are many ground wires on a atx cable, i've had a p2 350 running mounted on a piece of scrap wood for the last 4 years 24/7.. it just sits in my room in the open air.
as for the hdd, i'd recommend some L-brackets.
you can get buy with as little as 2 of them if you mount the drive vertically
do the same on the power supply, L-brackets.