trino BSR is much more than that new, though there is a small saving to be had for their current ex demo one. Still quite allot of money compared with your desired budget. Loosing the trino head saves quite a bit of money. The BMS are a better fit to your budget and better value but not as good quality. Even the slightly cheaper IMX is not bad at all (I have had one for years now)
Kyowa SDZ TR PL Stereo Zoom secondhand would be at the bottom of your budget and is a good Japanese value scope. I use one regularly, a good workhorse. Just make sure the zoom is not stiff.
Nikon (and Olympus) do nice stereo that secondhand would get you a trino in your budget. I like my long arm Nikon SM trino and they are reasonably plentiful and have been made for years now. See SMZ (2) nowdays for modern equivalent. I think I paid 500 / 550 (hefty trino premium
), boy is it heavy!!.
Every wants Trinos so you will always pay a substantial premium just for this feature. You may find you buy a stereo new or old and then add a long arm to it....as a long arm stereo trino is getting rarer to find!
Wild / Leica likely to out of budget.
thinking out of the box, Prior especially (but also Baker and Watson) do some lovely long arm stero scopes with fixed magnifications (generally 3 magnifications). These typically go from 30 pounds upto whatever they can get away with as a BIN on auction sites
but the killer is transport as they are heavy. If you can collect you can get an excellent on of these for 30 pounds perhaps upto 50. They are high quality and will get you going, allowing you to learn exactly what you might want to buy as premium later on.....with little outlay. They are then easily sold on or can for the start of your new microscope collection
might do a manual bid next time 
