Hi and welcome to Ribnet.
If you use the search function you will find similar questions coming up quite regularly. First off, there is a big difference between a 5 mtr RIB and a 6 mtr RIB. You need to decide what you want to use the RIB for, how many crew you need to accommodate and hence what size will suit your needs. If all things are equal a 5 mtr RIB will be less expensive than a 6 mtr RIB.
You might want to think about the fit out you will need, number of seats, and seating arrangements. What electronics are you after? Engines(s)? Questions, questions
You use the word "nice" in your description, this means different things to different people, but if you are meaning something close to as new condition you will need to up your budget considerably. If you are driven by budget, think what your minimum requirements are, and then search for them.
If you can provide more details, you might get a better response on here.