A tax residency certificate
is a certificate from HMRC to prove to the Greek tax authorities that you are remaining tax resident in the UK so you are not liable for tax in Greece, except on any income that arises in Greece. You will probably need to get an Apostille Stamp
on the certificate so make sure that it has a signature and it is of the person whose name appears, i.e. not pp by an assistant. You will also probably needed it translated but your lawyer/accountant should be able to arrange that for you.
Depending what happens with Brexit the worst case scenario is that you are limited to being here for 90 days in any 180 day period. Keep under that and you should be fine.
I would avoid a general power of attorney and go for something specific that authorises your lawyer only to do whatever you need him to do. I have heard some nasty stories of lawyers with a general power of attorney accessing bank accounts and using funds for their own purposes. Obviously not all lawyers are dishonest but not all are honest.
Purchase offers are like anywhere else. If you put in a low offer it might be accepted or it might not. Often Greeks are not in a rush to sell so they might well reject it, or even increase the price. Foreign owners are more likely to negotiate. The market is still depressed but sales are now going through. I would be surprised if you can't get 5 % knocked off and 10 % should be possible in some cases. You probably need to be a good negotiator to get more than that. As always it depends how badly you want that particular property.
Insurance with earthquake cover is available on all relatively new property but can be a problem with old property. I can't remember the specific date but I think it is some time in the 60s when building regulations changed. Greek policies tend to be a bit cheaper than UK ones. ERGO is pretty big and is underwritten by Munich Re so should be sound. Intasure in the UK does holiday properties but is a bit more expensive and is underwritten by Lloyds of London so again pretty safe. Check that it is still covered when you are only here for short periods. Some policies only apply if you are full time resident.