Its all depends where you will be searching on the beach or what type of beach it is.... generally if you want to detect along the shoreline or around wet sand then you will need something like a pulse induction machine, here is a more affordable model ie http://www.csmetaldetectors.com/product.aspx?pid=22Which types of detectors are suitable for beaches ?
If its dry sand then a fair few other brand models will be able to cope.
I use a Minelab E-Trac myself which copes very well on the beach in most situations but its a high end model and very expensive.
Cheers
mrix