Hope this can help you:
1. PAX (You mean PBX?) is just a small exchange that needs support from Publich switches to work, normally are name multiline equipment. PABX is a small exchange with more functions that makes a similar work like a public exchange but in the private side. (Hope it helps)
2.Sw is dimensioned using Routes (trunks), lines (if any), services, all this mean first of all the Topology network you want to design, after that next step is to look for a system that covers network topology and the performance you need (CCS, traffic etc)
Send me a email if further info is need lmartine@avantel.net
Tx