| When a top is caving in to the extent you seem to be indicating, this is usually the result of the bassbar letting go of the top, usually in the lower half. I've seen bassbars peeling away from the belly, sometimes the glue failing, sometimes taking a lot of belly with them, up to 75% of the bar's length. This is usually repairable, and the re-attached wood can then be reinforced using raw linen (NOT cotton!) strips across the bar as needed to hold the connection securely. This is to say heavy raw linen cloth on pre-warmed wood, the linen being saturated with successive applications of strong hide glue until it is hard, then polished somewhat for acoustical smoothness (reflecting sound more than absorbing it) and to avoid dust sticking to rough fibers. Such 'bandaid' fixes have held very well in my experience with no signs of letting go in at least a few cases in, so far, over a decade of professional use.
If some degree of arching correction is also needed beyond what merely gluing the bar back accomplishes, this is a much more expensive surgery. Even just cleaning up the joint and re-gluing and patching the bar is potentially expensive, as removing the belly, cleaning the belly/rib juncture, and re-attaching the belly properly is itself consumptive of several hours of skilled labour in most cases. Luthiers do get 'lucky' at times, when previous luthiers have left clean and just-right glued joints such that the glue separates cleanly without splintering of the belly edges, and minimizing the amount of preparatory work to re-glue it. This is rarely the case with cheaper instruments however, as they tend to use synthetic glues which multiply the labour input many times. I've had bellies of fine instruments part company with the ribs and blocks in less than 5 minutes. I've had tops of cheap Korean or Chinese basses take all day to remove, and much of the next day be consumed by repairing just the damage done to the edge in the removing, before even beginning the repairs for which the top had to be removed. A 'cheap' bass in such cases is anything but. On the other hand, some recent basses I've seen from China are a positive joy to work on, as they incorporate very fine traditional techniques and great woods. Several of the nicest model from a company using the 'J. Heinrich' label are every bit as well made as many lower-priced hand made American basses costing $12,000 to $20,000, though costing less than $3,000 in these cases. Hide glue throughout, beautiful joinery, and really amazingly fine wood, all beautifully carved and varnished. A number of other J. Heinrichs I've seen have been entirely different, awful things made using white glue and inferior materials, though the asking prices are similar, so this must be a large shop making diverse instruments to cater to different market tastes.
Anyway, hope this helps in your decision. I don't know the model you named so can't comment directly on its quality and value. | 
