Avionics upgrade - is this price reasonable?

Your list converts to about 40-50k in avionics plus cleaning up the wiring plus labor and a quote of 100k sounds as if the shop knows the specific airplane … I’d say quite normal and even realistic price quote given current hourly wages and shop loads, if the aircrafts avionics hasn’t been cleaned already during the last 5-10 years.

What are the hourly rates in Germany?

Over here this would equate to about 400 hours labor. Sounds a bit high.

The better avionics shops charge about 120€ +VAT, so let’s call it 140.

Impossible to say if the total is too high, not knowing what exactly has to be done.

Given the rule of thumb that the labour cost is approximately the same as the avionics cost it probably is within the expected ballpark.

The hardware you have specified can be obtained in European shops for €35k new, VAT included, plus a bit extra for wiring and hardware. You may be able to save on VAT by buying everything outside the EU and bringing it in a suitcase.
Labour costs will depend on the current status of your aircraft, but €65k (or even €40k) for labour seems very high in any event. If the existing installation needs only a minimal revision, installing all of the above will take some 160-180 hours. If your aircraft is on the approved model list of the relevant STCs, the engineering work shouldn’t take more than 50 hours. What type is your aircraft?
An hourly rate of €100-120 is OK for a maintenance organisation, but if you operate your aicraft non-commercially, the work can be done by a freelancer such as yours truly for a significantly lower price.

If you have a GNS 530, are you sure you want to replace it with a GTN 650 Xi? It’s obviously a matter of taste, but Avidyne IFD540 is a direct replacement for GNS 530 (it plugs into the same rack: take one out, put the other in), and, in my opinion, has a nicer user interface. By the way, why do you want GI 275 on the left but G5 on the right? Doesn’t seem very logical to me.
Your aircraft is a TB20, right?

Agree with @Ultranomad here. The 650 interface is, ahem, an acquired taste. Personally, I also hate touchscreens in light aircraft. Of course, your plane, your choice. Having overseen a few avionics upgrades in our club fleet, the cheaper quotes you mention sound more realistic to me.