A buddhist is expected to not belief antyhing "just because" : religion, wise person, father, friend, lover, or even the Buddha , tell you to believe it.
Gautma Siddharta Buddha said that everything he tells you must first see if it agrees with your better judgement, your experience and if after practiced you got the desired effects. If it agrees with your better judgement and your experience and gave you good fruit, only then you can accept it as truth. Never should anything be accepted as truth before it.
So yes, I would say a "good" buddhistt wouldn´t beleive in reincarnation, nirvana, etc, until he himself/herself finds it reasonable to his judgement AND has experienced some of it to say it is real.
A lot of people remember past lifes after meditation (to most it takes a lot of meditation) so before remembering it themselves, according to buddha, they have no reaso to beleive it.
In any case this is more or less the way I see it. A buddhist can be a monotheist in my way of seeing, if his edxperience points him to it.
It is actuall kinda hard, bec ause Buddha never talked of "buddhism" he talked about a method for getting rid of duhka(suffering). In these method, or even, methods, you take that which has helped you and leave that which has not helped you.
Buddha himself got enlightened that way. (according to the teachings at least
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