There some points you need to consider.
The bandwidth using G711 is about 83 Kbs. This does not include any overhead added by your WAN link. You still need some bandwidth for signalling. If you use presence features on the phones the signalling bandwidth needed can have large peaks.
Who is supplying the phones? If you supply them you need to check if they are supported by the company hosting the VoIP server.
If you are using new phones the Cisco 3550 POE switch may be a problem as its pre 802.3af, the standard for POE. I do not think some of the new phones will work with this.
Do the PCs and laptops that will be connected through the phones need Gigabit connections? Most of the IP phones that have the inbuilt switch only do 100 Meg.
What budget have you allowed for the phones? IN the UK the SPA504G is about £90. Some places will sell refurbed SPA942s for between £40 and £60. Even at the cheaper end of this that £1600 for the phones or about $2600. IP Phones, especially POE ones, are expensive, and if you find some cheap there is usually a reason. Do not touch Grandstream phones. Many of the older Cisco 7960 and 7940 phones come with SCCP, and not SIP. Getting the SIP firmware will need a support contract, its not available as a free download. Cisco do the SPA303 which has a switch so can be used with 1 cable to the desktop, but its not POE.
The router used will need to be able to prioritise the VoIP traffic on the uplink. If possible check with the ISP and see if VoIP traffic on you WAN link can be prioritised. This is available in the UK from some ISPs.
I have used the Cisco SMB SF 300 24 port and 8 port versions of these on quite a few VoIP installs, and recommend them. There is a 48 port version of this switch but I have not used it. In the UK the SF 300 24P POE switch is about £300 (about $480). The HP and Netgear switch usually work out more expensive. The SF 300 range are 100 Meg switches with Gigabit uplink ports. These work fine with the Cisco SPA500 range of phones, the older SPA942, Snom 300 and 370s. The are managed, have various different Vlan options (trunking, access, tagged and untagged), very good QoS, and if needed will do static routing at L3. I honestly do not think you will find something you need to manage that you cannot with these switches. The warranty is good with advanced next business day replacement.
If you really want to do this on the cheap buy some headsets and use a softphone on the PCs/Laptops. This will be on the same LAN as the data. Providing the SIP and RTP packets are correctly marked and the QoS on the switch is setup correctly, then the softphone and the usual PC traffic can work together OK. Doing things this way would save a great deal on the switch as non POE switchs are much much cheaper than POE. In the UK a POE switch costs more than twice as much as the same model non POE.
Rob.