That's great news. Unfortunately many bike shops are more interested in making a sale than making sure their customers get a bike that's suitable for their needs and fits them. It sounds like the place you went to is not out to make a quick buck, and because they exist to provide an opportunity for prisoners to learn a useful trade and skill, there is probably more likelihood of the bikes they sell being in good working order than some retailers of new bikes (the quality of mechanics/servicing at some of the big retailers can be highly variable, and new bikes are not always properly assembled, e.g. it's not uncommon to find grease was not used when assembling a new bike, with the result that things like pedals and seatposts become seized and impossible to remove).
Hopefully the place you visited will have a bike that ticks enough of your boxes if you keep going back over the next few weeks to check. The most important thing is that the bike fits you. However, there are some 'nice to haves' which you may want/need (even if only in the longer term), which might influence your choice if you end up with the luxury of having to choose between more than one bike, or which you might be able to get more cheaply when buying a bike:
- Mudguards. As I've said, you can get by without them in spring and summer while it's warm and dry, but the more you ride in winter and on wet/muddy roads and paths, the more you will want them. It's possible the bike you choose might even have them fitted, but that's unlikely because you want an MTB, and they are often not fitted to MTBs (they tend instead to be fitted as standard more on bikes with narrower tyres, like hybrids and utility bikes). However, as long as an MTB has mudguard eyelets you can fit them (which is more likely on a non-suspension MTB).
When selling a new bike most bike shops will fit mudguards free of charge if the mudguards are bought at the same time. The bike recycling place probably does not sell accessories like mudguards, but they might be willing to fit a pair of mudguards for you if you buy them from a shop and take them there. Mudguard fitting is not difficult, but it is a fiddly job, so if you do not feel comfortable doing it yourself and they will offer to do it for a small extra charge, that would be something to think about. Note that you need to make sure you get the right size when buying mudguards (both for the tyre diameter, e.g. 26", 29" or 700C, and the tyre width - MTB tyres need wide mudguards).
- A rear rack. Once you start riding any distance, or if you ride to the shops, you will want to be able to carry stuff. You can use a small backpack or similar, but it's usually more comfortable and more practical to have a bag on the bike, which means you will need a rear rack. Although you can buy panniers and other purpose made bags to fit on a rear rack, to begin with - providing you are not carrying too much - you could probably get away with almost any bag and just use a bungee cord to strap it to the top of the rack. This will allow you to carry a jacket or spare item of clothing (which you can then take off/put on when you get hot/cold), some food like sandwiches, something to drink, tools, and a lock (Bristol has a poor reputation for bike theft).
Although you can fit most rear racks yourself quite easily, it's possible that the recycling place might be willing to fit one for you. I say that because they are recycling many bikes, and some of them will have racks fitted. If you get an MTB (which is unlikely to be fitted with a rack), they might be willing to take off the rack from one of the other bikes they have in stock and fit that to the MTB. If they will offer to do that for a small charge, I would do that because a new rack will probably cost upwards of £20.
Finally you will also need some very basic tools and spares, which you will probably need to buy separately/from a shop:
- One, but better two, spare inner tubes (buy the correct size for the tyres fitted to your bike, with the same valve type, i.e. presta or schraeder. This is something which the recycling place might sell, so they would make sure you bought the correct ones (or they could tell you what size/type to buy from a shop).
- A multi-tool, e.g. like the
Topeak Hexus, which Wiggle has currently on offer for £12.99
- The Hexus comes with a couple of tyre levers, but they are not great, so it's best also to buy a separate set of tyre levers. This set of
Park tyre levers plus glueless patches is good value for money, and if you buy them together with the Hexus, you get free delivery from Wiggle. The glueless patches are not good for permanent puncture repairs, for which you need something like
this, but they are excellent for an emergency on the road if you've already used your spare inner tube(s). Youtube will give you some videos showing how to deal with a puncture on the road (when it's usually best/quickest to replace the tube with a spare, rather than patch the tube).
- A pump. I would wait until you had bought the bike before getting a pump. What pump is best/suitable will depend somewhat upon the bike and tyres/tubes it has.