1. 4 string or 5 string
4 string standard tuning ( E A D G )
5 string standard tuning ( B E A D G )
there is only one difference (aside from physical differences) between 4 and 5 stringers - 5 stringers give you lower notes. there is no difference in transposition between 4 and 5 stringers that is a result of the presence of the low B. transposition usually is only an issue on instruments with different tunings. different tunings will cause this issue regardless of the number of strings the bass has.
now, for me, having access to lower notes is not a good enough reason for one to get a 5 stringer. it's like saying the main benefit from a 6 stringer is being able to play lower and higher notes due to the low B and high C strings. that is not the case.
the most important reason for having extra strings would be to have a wider tonal range without having to go up and down the neck. and when would one need a wider tonal range? in 2 cases - (1) for playing chords and (2) for soloing.
2. brand new or used, cheapo or high end
there have been alot of grandfather stories and conspiracy theories regarding low end basses, and basses made in certain countries. as such, i will put it in this way:
basses made in certain countries are less preferred by players because they usually have a greater variance in their quality. meaning to say, it is more common to come across a lousy china made instrument than an american made instrument.
with this in mind it is harder for an inexperienced player to identify which basses are lemons and which are gems.
this is reason why a 'good bass' can last longer than a poorly made ones. also bear in mind that some very experienced guitarists in these forums are champions of basses that many would consider obscure or cheapo. i'm not trying to undermine their expertise, but it is also worth noting that often the instruments that they speak very highly of have probably not been owned by them for long periods.
so far i have explained why a used bass would still have alot of value, and you already know that it's easier on your wallet. there is another reason why you should get used;
get a used bass, learn how to take care of it. it's less of a heartache if a used bass gets a ding or a scratch due to your inexperience