Hi,
Any MIDI module wil respond to a standard set of MIDi control messages including Hold. However, for the life of me I cannot think of any MIDi module with it's own Hold switch.
If your pedal unit features a hold switch of it's own it wil work just fine. However, you wil still need to hold the Hold switch down in order for the notes to sustain. Unless you can re-configure the hold switch to work otherwise (You'll need to check the user's guide for that).
To be honest with you I can't think of any dedicated cheap MIDI modules designed specifically for Bass sounds (Aside from may be Novation Bassstation. But unless you're after techno type synth bass lines it's not your synth).
There are some older (since you want cheap) MIDI modules like:
1. Alesis QSR, Nano synth. Also QS and Quadra series keyboards. They don't cost a ton of money and the sound is decent by even today's standards.
2. Roland Jv series (880, 1080, 2080) if you can find them cheap. Also, if you can get one with Roland's Basses and Drums wave ROM expansion board (or you can buy one separately and install it yourself). Great for bass. Great for many other sounds too.
3. Korg X3R, X5R, O1R/W, O3R/W, Trinity rack series. Decent sounds. Nothing spectacular though. Just like any working man's keyboard/module with pretty generic set of sounds. Will do bass alright.
Since I can only speak about synths that I've used myself (the list obviously goes on) will have to wait 'til others post more.
There are obviously other ways to approach the whole thing. If your computer is good enough to handle the task, Spectrasonics "Trilogy" is a dedicated and well designed all-bass virtual software instrument. Of coarse for that you wil need a decent audio card, some sequencer/audio software capable of hosting VSTi or at least a standalone VSTi host application if you don't care about recording and MIDi sequencing.
Whatever you will end up getting keep in mind that synthesizers and synthesizer workstations aren't made to be amped with guitar and bass amps and require a high quality stereo connection. If you haven't got a mixer and a pair of studio monitors around a home Hi-Fi will do the job.
Not that I'm saying that hooking up a synth to guitar or bass amps is a really bad idea but it's only useful if you want to add a very special and muddy character to the sound. Not a standard practice I should say. very "guitar geek" cheap ass kind of stuff.
Worked for me when I didn't have anything but an old ARP, a TL and a couple of amps. Back in the days when beer seemed expensive....
-ED-
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A gentleman is one who never hurts anyone's feelings unintentionally.
- - - Oscar Wilde