For winter use you need tires that are relatively soft and supple. If more than 1-2 years old used tires will become hard and the difference in traction is huge. 6mm is marginal for winter use. Most new winter tires have around 9-10mm tread depth. Bearing in mind that you will be using these tires for 2-3 years I would recommend buying new.
Do not put winter tires on your present rims unless you plan on leaving them there and finding new rims for your summer tires. The winter salt will be hard on your rims and tires do not fare well when dis-mounted and mounted more than a (very) few times. It stretches the bead which gets less flexible with age and the potential for damage during the whole process is increased.
If possible use Porsche OEM wheels or a recognised TUV certified aftermarket brand. You may not be driving quite as vigourously on your winter tires, but you still need a rim that is built as well as your Porsche and can stand up to the rigors of winter use.
I wouldn’t worry too much about getting compatible TMPS. Buy yourself a good tire pressure guage and save the money!