Hmmm......... have we been listening to what we are reading, people? If all you want is preserve printable copies of old drawings, then put them in Adobe pdf's. Then anyone with a computer will be able to print them forever. If you want real 3D models with anything like accuracy FORGET SCANNING!!!!!! This is of course an oxymoron, because if you are making solid models, you NEED PERFECT ACCURACY. If not it is going to come back and bite you in the @$$ VERY VERY HARD one day soon. That is expensive - much more so than doing it right in the first place. If you can't afford to do it right you are either underfunded, in the wrong business, or serioulsy and unrealistically under bidding your work.
Here are a FEW of the horrors of scanning.
1. Paper, even mylar suffers from non-uniform expansion and shrinkage with temperature, humidity, age, physical handling, etc.
2. Plotters do not give you perfectly accurate plots in the first place - certainly not accurate enough to create solid models from.
3. I defy you to get the scan perfectly square to the plot - even with the magic software. C'mon, the original isn't that straight anyway.
4. Scanning is raster for crying out loud. Is goes in finite incremental x/y steps!!!
5. Line weight/quality - where is the true center (and ends) of that line?
6. Do I need to list more..........?
Now, sure you might find an "apparently acceptable" work around for some of these things (remember to wear your kevlar underwear, though) but you are never going beat them all and you are going to invest large amounts of very frustrating time and money in trying.
Rebuild the models manually (and thus from them the drawings if necessary) on an as-needed basis.
Be naughty - save Santa a trip.