No. All PIX's are end-of-lifed. No licenses upgrades or smartnet is available for them. The ASA's have been out for quite some years, thats the currently supported hardware.
Restricted as in you don't get all the features, like no HA failover. On this particular one, it looks like unique hosts are unlimited, but other models could be restricted to 10 inside hosts, or
50 inside hosts.
You could add-on interfaces to a 515 and up. But all PIX hardware went EOS two years ago.
The ASA5505 is the current replacement, and you can easily find better deals with online stores than what eBay has. (and for used gear, most certainly find better deal from hardware resellers even on ancient EOL'd hardware than on eBay as well).
Lots of other vendors give you alot more bang for the buck than Cisco on firewalls though, with Enterprise level reliability.
I'd look first at FortiGate for firewalls, and then Juniper. Both have products that fill in quite alot between the ASA5505 and ASA5510 of Cisco and offer a lot more features.
If you are looking near the low end, the FortiGate 50B has dual WAN, can handle full wirespeed (100Mbps) firewall, and if you want to pay for some more of the subscriptions, has IPS, antivirus, antispam, etc. You can also setup HA by just buying another without additional licenses.