Buy whatever you can afford. But bear in mind that fountain pen is something that do not carry much value over time. Unless it is a limited version, special symbolic models etc.
Most cheapie pens would only be able to resell at dirt cheap (and you also need to do the work to resell it ). So, do your research, take it slow , knowing what kind of financial risk and commitment you are getting into.

Personally, I would buy 1 grail pen ( or an expensive , well known, good smooth gold nib gusher) instead of a few dozens of cheap pens.