Take a look at Talkswitch,
http://www.talkswitch.com/Fairly basic phone system that includes all the features you are looking for. The base unit and phone sets are reasonably priced and it is simple to install yourself. It will work with 3rd party phone sets (analog and IP) as well - though for the paging feature I think you would want to use their phone sets.
The nice thing is that the system is hybrid and works with both analog and IP phone sets and trunks. It is also upgradeable so you can add lines and extensions. The very base model handles 2 analog lines, 4 analog extensions and 12 IP extensions. It also recognizes fax calls and redirects a fax from any line to the fax 'extension'.
As the first poster has mentioned, many of the phone systems which offer the features you are looking for have significant price tag. Likely around twice the cost of the Talkswitch equipment or more.
Asterisk based phone systems can also be quite inexpensive but are not always user friendly for configuration. You could take a look at Trixbox for DIY or for a fully commercial Asterisk based phone system, Fonality as an example. Asterisk does scale up well so any system is likely not to need to be upgraded but you would need to get analog interface cards for analog lines or if you want to use analog phone sets.
http://www.trixbox.org/http://www.fonality.com/Another option which is quite inexpensive up front is to look into 'hosted' or 'virtual' PBX arrangements. Many voip companies offer business packages that provide most of the functionality of a full blown PBX with no equipment on your site other than the phone sets. Here is a link to the type of operation I am referring to,
http://www.virtualpbx.com/The strengths is a virtual setup is very dynamic so fast growth is easy to accommodate without big initial costs. This kind of PBX arrangement is often pretty attractive for small startups.
