Buying of a type rating aside and answering the question posed directly........
As stated, plenty of pilots have flown the 757/767 as their first type. The passing of your first type rating on something bigger than a piston twin comes down to basic ability and an awful lot of hard work.
I would say that any twin jet of the 737/A320 size or larger is on the 'complex' side of the type rating fence and all will require large amounts of determination to pass. It was certainly the most difficult course that I did during the whole of my flying training, but equally the most enjoyable and rewarding, especially as someone was paying me to do it (novel concept I know!)
People do fail though and many need extra time, which equates to extra expense, hence why airlines try so hard to select pilots who they think will not only pass, but do so in as close to minimum time as is allowed.
Like I have already said, you have to have the basic ability to handle the workload (both technically and the sheer volume) and attitude is one of the biggest factors. If you start to struggle, make sure you seek your instructors help and put in the extra work to catch up. Usually the program is so intense there is little room for going over things again and again. In my experience though, even if someone is struggling with something, if they are showing the right attitude and trying their absolute hardest the trainers will do whatever they can to get you through, it's the people who let it get on top of them and start to sink making all the excuses in the world who eventually leave the instructors with little choice but to recommend their removal from the course.
I wish you well with whatever you choose.
PP