Well for what is worth here goes my opinion.
I´m on the same situation as yourself and what I state is not from own experience but from hours reading and putting together my own opinion formed on the knowledge of hundreds of stringers, more and less experienced.
At the present time one of the best values in the market, for a dropweight machine is without question (in my mind) the stringway brand. The ML90 and ML100 must be, quality/price standard, the best value in the market. I started my quest with a challenge I in mind and ended stretching my budget for a M90, which Mr. Fred kind of transformed my order to a ML90 without any further additional cost. It comes with a double and the famous triple flying stringway clamp (for just 10€ extra) which by now you already know it is the best brand quality flying clamp in the market. With shipping costs included I spent little more than 400€ in total. You are considering a Challenge II with possible Stringway flying clamps (which I advice strongly) which would be with shiping costs included maybe 100€ diference from going stringway all the way. In the long run I believe you will be sorry not to have spent a little more and get great quality for everything and not only extra flying clamps that are adapted to a lesser machine. Plus visit their website and learn about the stringway tensionhead which is a major improvment over any other Dropweight tension head. The company is original from Netherlands and I strongly recommend that you contact them by email and Mr. Fred will surely answer you swiftly (he has been spectacular with me!) and help you with everything you need to know. Mr. Fred is a wonderfull person with a vision of service customer and real honest advice that is rare today. By my own experience I can tell you that he won´t sell anything to you that he believes won´t be necessary (I wanted an extra expensive product and he provided me with his knowledge backed up by literature showing me that it was absolutely no necessary for now... his atitude was priceless to me). Plus he will do absolutely anything in his power to give you the best price he can for what you wish to purchase. I know he did for me, giving me good prices under the ones rated on the website.
I´m waiting for it to arrive and will for sure post my believes and experiences that I really believe will be nothing else than positive. I ended up spending a little more than what I wanted in the first place, but as you find out on your quest for knowledge... if you want quality you just can´t go the cheaper way. I repeat once again for the quality, stringway prices in Europe are a steal and matched by no other company. This is my honest belief and my contribution to your curiosity in finding a little more around here about stringway... not only with flying clamps but also with their machines... which by the way have a 10 years warranty (I think this says it all about their product quality).