I have "retrofitted" heated seats in my C-class (exchanged the complete cloth interior for a C36 AMG interior) and have to say that it can be done (in a fairly easy way).
I think you are considering to retrofit the heated seats in your 190...If you can get an used system from any W201/W124 or even W202/W210, you can easily fit it into your 190.
The heat exchangers are stuck in the upholstery of the seats (you need to dismantle the seats to put the heat exchangers in) and the electrical system is to be considered as being simple.
If you can get some used mercedes parts, it can be done by a non-mechanic within a day...
(by the way: did you check the ET size of your wheels?)
with a minor 600+k