The Raspberry Pi probably won't be of much use to your company for the things you mentioned. I couldn't find anything on the Tally website that suggest they have a Linux version, and even if they did, they probably would not have a version compiled for the ARM processor the Pi uses. MS Office will not currently run on the Pi, but it might when Windows 10 is released - Microsoft is planning a version of Windows 10 specifically for the Raspberry Pi. In the meantime, you could run LibreOffice or OpenOffice on the Pi. There are several web browsers available for the Raspberry Pi, so that will not be a problem. You should be able to find plenty of places on the web to buy a Raspberry Pi. I got mine from MCM Electronics. I bought a starter kit which includes a power supply, case, wifi dongle, and a few other items. If I remember correctly, the starter kit cost me 55 or 60 dollars. Well worth it. You should probably buy the RPi 2, as it has a faster processor and more memory than the Version 1 models. The Pi is cheap enough that you could just buy one to play with and see if it will meet your needs; they're only 35 bucks. If it won't work for you, well, you're only out 35 bucks. In the U.S., the Pi has a one year warranty.
“So you say you love the poor? Name them.” - Gustavo Gutierrez