The answer is yes. Below are the system requirements for version 2015 & 2016
Sage 50 Accounting 2015
U.S. Edition
System requirements

Mult iuser
• Multiuser environments are supported in Sage 50 Premium Accounting and higher
• Multiuser mode is optimized for Windows Server 2003, Windows Server 2008, or Windows Server 2012 client-server networks,
and Windows Vista, Windows 7, or Windows 8 peer-to-peer networks
• A maximum of ıve licensed named users is allowed for Sage 50 Premium Accounting and 40 named users for Sage 50
Here is the link
www.sage.com/.../SystemRequirements_Sage50US

Yes, you can set up a peer to peer network using windows 7. Server 2003 will not even work for 5age 50 2016.

I would go with at least 8 gigs of ram, A very fast processor, (Look at a gaming machine) and at least 500 gigs to one terabyte of hard drive memory.

Another alternative is to have your data hosted. The cost for hosting Sage 50 is about $50 per user. (actual cost depends on what programs you want hosted and how many actual users you have in the system)

If you are on version 2016 then the Windows 2003 server will not work. If you are trying to save money you can set up a Peer to Peer network using Windows 7 or Windows 8. A server is configured to run in a network environment so a server is always the best choice to run your network. However, I have many small business that run Sage 50 on a Peer to Peer network and it serves their purpose. For the Peer to Peer network environment, you can either purchase a new computer (not a server) that is beefier than the Sage's recommended configurations (below) to use in place of your server and connect the other workstations, or you can choose one of your existing workstations to be a server /workstation. If you choose the server/workstation where a User is going to be using that server/workstation, then that User must be very careful to never shut off or reboot their computer while the other workstations are in Sage. Doings so could seriously corrupt the Sage database. Also, in the server/workstation environment, you may have an overall slowdown across the network because the Sever/workstation is processing all the other workstations requests and well as processing the Users requests on the server/workstation. This speed issue becomes a noticeable issue with large Sage databases, the ram and processing speed on every workstation.

Hope this helps you.

Here are the system requirements for version 2016.
Here are the system requirements from the Sage knowledge-base for Sage 50 version 2016.
System Requirements: Sage 50—U.S. Edition 2016 Article
Products
Sage 50—U.S. Edition
Minimum requirements

2.0 GHz processor for single user and multiple users
1 GB of RAM for single user and multiple users
Windows Vista® SP2, Windows® 7, or Windows 8, with the latest updates from Microsoft installed
1 GB of disk space for installation
Internet Explorer 9.0, 10.0, and 11.0 supported
Microsoft®.NET Framework 4.5.2; requires an additional 280 MB to 610 MB
At least high-color (16-bit) SVGA video; supports 1024x768 resolution with small fonts required
DVD-ROM
All online features/services require Internet access
Excel®, Outlook®, and Word integration requires Microsoft Excel, Outlook, and Word 2007, 2010, or 2013 (32-bit)
Printers supported by Microsoft Vista, Windows 7, or Windows 8
In-product demos require Adobe® Flash® Player 10 or greater
Adobe® Reader® 9.0 required
Multiuser environments are supported in Sage 50 Premium Accounting and higher
Multiuser mode is optimized for Windows Server 2008 or Windows Server 2012 client-server networks, and Windows Vista, Windows 7, or Windows 8 peer-to-peer networks
A maximum of five licensed named users is allowed for Sage 50 Premium Accounting and 40 named users for Sage 50 Quantum Accounting; a named user account is granted a license when selected in the user maintenance screen
1 GB of disk space for installation of components on server
Windows Server 2008, or Windows Server 2012 along with Remote Desktop Connection or Remote Desktop Web Connection client is required to run in a Windows Terminal Services environment; no more than 5 named users for Sage 50 Premium Accounting or 40 named users for Sage 50 Quantum Accounting
Terminal Server requires additional memory when more than one user is running under Windows Terminal Services
Customer registration and acceptance of License Agreement for Sage 50 Accounting software products

Recommended configuration
2.4 GHz processor for single user and multiple users
1 GB of RAM for single user and 2 GB for multiple users