We are building one just now - getting the concrete base poured next week if the weather is good, it'll have the same purpose as your's will mojo and the base is a size 18m x 6m so approx size with yours more or less. Timber frame is the way to go if you want it up quick, we're getting a joiner in to put up the main frame and the roof trusses and then we're doing the rest. That way it shouldn't cost much more than £5k including materials. Saying that we haven't designed exactly what it's going to look like yet, we just want the base in before the bad weather starts (shhhhh......) and then we'll do the rest in the spring, unless my OH has a must do moment. Timber frame is super quick and looks nice, we're fussy architects so don't do the commercial farm building thing, it's beside listed buildings here so we want something that looks fitting. Would be interesting to know what you will pay for that, a near neighbour paid something like £8k for a big big size, concrete block base, timber top and tin roof, quite reasonable I thought.