Well, if all of your old software is "DOS" based, even though DOS is long gone and extinct, running DOS programs is not.
Instead of trying to downgrade, you can look at this program: DOSBOX (opensource) available at this web link: http://www.dosbox.com/
Before doing anything, though, go through the tutorial they have on the web site and also look at any other information you can find on it before downloading/installing it.
This program first came out for Linux and is popular but they did make a Windows version also and have a "Windows installer".
Also note that the DOS version maybe too low for your DOS programs to work. That was a problem with it and it emulated DOS 5.x and not 6.x. It is opensource and free, so, there is no cost in trying it.
Your only other option is to find an off-lease laptop or desktop with Windows 7 pre-installed on it and use that machine instead of one of these tablets. There are no drivers for all components for Windows 7 for the Surface line. Windows 7 does have most
generic drivers but not for everything.
As for an off-lease machine, you are going to have to hurry up in that option; they are currently being upgraded to Windows 8.1. Windows 7 is history and is supposed to have been removed from the market. There are a few places still selling it (leftovers).