In my experience, there are two essential features that you need from a preamp for double bass - a high impedance input (10MOhm) and a high pass filter (HPF) to roll off the really low end frequencies which will really help in the battle against feedback.
The Fishman and K&K preamps both have 10MOhm input impedance, but only the Fishman units have HPF controls.
I find that with a good preamp with HPF, your choice of amp is much less critical, so in your position I'd go for the Fishman platinum pro EQ pre and then look for a second hand combo like a TC BG250 208 as suggested above.
The Fishman pro EQ is all the preamp you'll ever need, unless you start using two pickups or blending a pickup and mic, so it will see you well along your playing journey.
If you still want to squeeze the pennies, then look for the previous version of the Platinum pro EQ - the Platinum Pro. These can be found used for around £120 and has the 10MOhm input, HPF, 5 band EQ and a DI. If you're really lucky you might find a used Fdeck HPF or similar clone for about £100 - this is a real bare bones preamp with a high impedance input, gain and variable HPF, in a tiny box that can be mounted on your tailpiece.