Recognise that being a landlord doesn't just involve receiving the rent, but involves significant costs so that you can provide the service tenants are paying for, plus it requires work and time too - some of that can be reduced by paying an agent to do some of the work, but you need to expect to still spend time on it.
So, you must expect to get the property up to standard to start with and to meet the regulations for EPC, Gas cert etc. Recognise that if you present a quality property you will get quality tenants more easily, but shabby attracts people who won't look after it.
Expect to fork out for repairs - a tenant is paying for the property to be fully maintained all the time. You can't decide not to bother with a repair or to take 3 days to call a workman as you might do if you live there yourself. They have paid for things to be repaired speedily......and doing that can cost quite a lot - you have to be willing to pay for that speedy service - it's not doing the tenant a favour and being nice or kind, it's simply what they have paid for. A good landlord is someone who meets all of their obligations quickly and and to a good standard - it's not about being friendly, but delivering what they have paid for.
Know there is no cutting corners and nipping off fast abroad. You need to fully familiarise yourself with the obligations of a LL even if you have an agent. You need to be willing to put time in from abroad and to ensure the agency do everything to a goodbye tankards suchbas inventories, regular checks etc and follow up any issues. You will have to check they do these things.
Do everything properly and by the book - get your mortgage transferred, ensure you keep records for tax, make sure all legal obligations are met, ensure all credit checks etc are done. Being a LL is a business transaction and not a hobby, so be prepared to put time and money in - it's not a quick and easy way to make money or to offload a property while you're away with no effort.
Realise that the property becomes the tenants home and they must be allowed to live in it as their home. That means, no harassing them or visiting or setting daft requirements about cleaning etc. However, do insisit the agency carry out regular early inspections so that if the tenants aren't maintaining the property and more than wear and tear is occurring, they can follow it up and if necessary serve notice. It is at the end of the tenancy that you can recover any damages carried out which go beyond wear and tear. Know that at the end of a tenancy, fair wear and tear cannot be claimed for - so carpets being rather worn, or a few scuffs on the wall are to be expected and part of the LLs maintenance costs, not to be funded from tenant deposit. More serious actual damage can be claimed but deposits are small these days - so if there are early signs of damage to property or non payment of rent, do be prepared to serve notice in accordance with the law, rather than letting tenancies drag on. Expect to have to spend money between tenants - decorating is needed more regularly in rentals and often between each tenant. Carpets might need replacing every 3-5 years and you'd expect to spend on kitchens and bathrooms every 5 years. It's no good feeling cross about these costs but you must expect them as part of the business costs and ensure you can pay them. Don't rely on an agency being able to tell you exactly what needs doing being lets.
So, top tip, realise it will involve work, there is no skipping off abroad to leave the property to rake in the cash while you ignore it. People who pay are paying for a service and it is your responsibility to provide a well maintained property and ensure any defects are sorted in a very speedy fashion, regardless of cost to yourself. And don't get into it if you aren't really prepared to do it properly - why should someone pay for a home and then receive a shoddy quality because you are abroad - quite where you are isn't their concern, but they should be able to receive the full service they have paid for regardless.