With reference to whether you get a Personal Allowance, please google this and look at the first item displayed "Tax on you UK income if you live abroad: Personal Allowance - GOV.UK"
The biggest change in recent times for UK by to let purchases is the ability to claim finance deductions, it use to be possible claim mortgage interest but that is now being wound back and being replaced by a tax credit, currently, this is in the transition period.
As for management companies, probably best to have one of the local agents in the area look after it, perhaps even the agent you buy from could be a negotiating point on their management fees. They all charge typically around 10% or more for full management, less if you just want them to do the rent collection. Normally this includes regular inspections of the property, limited basic maintenance without your input and more involved maintenance with your consent. The problem is that you will never know how good they truly are until you have problems whether it be with the tenants or the property itself.
When your looking for a place, put things like the big-ticket items such as windows/doors and boiler at the top of the list of needs, should look for newer installs as that should give you some clear time without any significant maintenance costs. Also another tax consideration only maintenance not improvements is considered an allowable deduction. Say if you brought a place with single glazed windows and then replaced them with double glazing, this is an improvement and would not be an allowable deduction. But if you already had double glazing and they needed replacement due to the condition of them it would be considered maintenance and allowable.
Finally, another new requirement to consider, rental properties have to have an energy performance certificate of E and above, so something to consider when selecting a property. This was effective from 1st April 2018.