Not a race here, but I bought and apartment in Lugano in 2003 for 140 rents.
Going back to the OP, buying in the current interest rate climate is usually much cheaper than renting. Bear in mind though you will be responsible for the full share of the building expenses. You will also gain on any capital appreciation.
It should also be possible to borrow 80%, but the bank will give a second mortgage for the additional loan but at a higher interest rate. In the case of Raiffeisen, 5 years fixed at 1.04% + 2.8% for the extra. The interest is atx deductible but you'll pay a tax on the value set by the tax office. In Ticino it's 1/mille but on a value that is about 20% of the real value.
If your plan is to refurb and sell, assuming the numbers work, you'll also have to pay quite a high tax on the gain unless you keep it for 10 years.
From the owners point of view, to do a full refurb will likely cost in the region of CHF 100,000, which equates to about 5 years rent, longer if they nett less after costs and interest, so this is perhaps why they do not consider a full refurb but just tart the place up after each tenant.
When one case closes another opens! Maybe with this info you'll go on to be a property owner in Switzerland. Wish you all the best whichever route you take.
I would suggest contacting the property owner directly to inquire if they are considering selling. At the very least, you could ask them to please first consider you if they do decide to sell.
Thought that might be the case- many of our Swiss friends own a large amount of properties as investment and to reduce taxes- and it would not be in their interest to sell piece by piece.
If they rent out through an agency, and rent out multiple objects the only option to buy is to overbid hugely which is not in OP's interest, and besides that, if they own the building there is not a single need for them to start bothering with a co-owner.
And tbh 500K for a 3 room object in Zurich next to sihlcity sounds ridiculously cheap.