A lot cheaper is to simply live with the standard controller and live the limitations wich is ok in my eyes... want any real speed you still need SCSI anyway me think. Get hold of a 2.5" to 3.5" cable or make one yourself by connecting the first 40 lines from the 2.5" cable onto the 40 lines in the 3.5" cable (leaving out the 4 last cables supplying power to a 2.5" HD, line 1 is usually marked in some way btw).
An Ultraslim 3.5" HD fits in the case by only having to remove the small shield covering the clockport/chipmem, want a bigger HD you need to either cut the shield or simply remove it all together. Fixed position is almost impossible for these drives (I'm still looking for a way to do it nicely with the shield off). And you need to put the HD in an antistatic bag.
