Sorry never used a third party but have done my own. Cost and time depends on whether the drawings are dimensioned or just to scale (metric or old?) and how complex and of course how many. Appreciate you are looking for a recommendation and not just Google it! May also be worth scanning part of the plans to give folk an idea of details etc. Also do you need specific levels/layers, colours etc?
If you only need a few email me, if you have loads keep looking on t’web, if loads get scans sent to India, seems to be a way, but specify what you need back, layers would be important to me. Good luck.