I suspect that the guitar compressors aren't ideal for two reasons:
likely to be optimised for guitar usage w.r.t. the level of signal they expect and the
range of their controls. Stomp box compressors are usually used to funk up rhythm parts or
increase lead guitar sustain.
2) They'll be mono, and your overall mix will be
Personally, if budget is really tight and you just want to see what
difference you can make, I'd have a look on eBay for an Alesis nanocompressor, really
cheap stereo compressor but pretty versatile. Make sure you get one with a PSU because the
Alesis PSUs are AC and can't easily be replaced with a generic one from comet / radioshack
If you want to spend a little more, there are lots of 19'' units in
the £80 to £150 bracket. Rather than list any, look at the online retailers who
advertise in SoS. It might be worth thinking about buying a cheapish channel strip instead
if you record with microphones a lot, as these often have a (mono) compressor built in.
The now discontinued Focusrite Penta was unusual but well regarded stereo compressor with
a mono mic channel.
If it's a tape based unit, there's a little bit of
compression already if you record at a high level.
If you're handy with a
soldering iron, there are several DIY compressors for guitar or studio usage that can be
built for about £10 - £20 parts, plus a power supply and a case to put it in.
I can't vouch for
any of these, i've not yet built any myself.
As a compete alternative, you
could look at using some cheap / free software on your computer to see if you like the
difference that compression makes.