Air heat pump or use gas and solar?

EG 90sqm OG 86sqm 1m knee wall, open roof 4.60 ceiling height, Kfw 40 gas connection 2150€ Controlled residential ventilation Zehnder with heat recovery Heating complete 6200€ Viessmann Vitodens Including basic fee 411€ 2014 392€2015 402€ 2016. Maintenance 95€ House price 258000 turnkey upscale equipment plus 18000 base plate

Apparently, there are also subsidies for gas / solar.

Google Bafa

500 € for a solar collector system

It also mentions something about gas-powered heat pumps. What is that supposed to be? A regular gas boiler? For that, there are €4,500.

The subsidy only applies to existing buildings (at least 2 years old). For new buildings, this only applies to large solar systems (e.g., in apartment buildings).

We have now received feedback that we can install the originally offered gas and solar solution. We will then commission it accordingly. We find it better than the experiment with the [Sole Wärmepumpe]. Especially since the underfloor heating is designed in such a way that it can be switched later in case of emergency. Although it will probably be difficult with the drilling then.

One more question:

As a hot water storage tank, the Viessmann Vitocell-100B was offered. It is made of steel with Ceraprotect enamel coating.

I saw on the Viessmann website that there is the Vitocell 300-B, which is additionally made of corrosion-resistant stainless steel and is rust-free. Wouldn't that be the better choice or unnecessary in my case?

Surely that's better. But if you google it, you'll find that the 300-B is already twice as expensive as the 100-B when bought online. For example, €2000 instead of €1000 for the 300l model. Therefore, in my opinion, it doesn't make sense. The 300-B is available "only" up to 500l.

How long does the non-rust-resistant tank that was offered to me last? At some point, it will probably rust away on me.