fix the roof now. Fixing things while tenants are in involves finding a convenient time for access and arranging that with tenants, trades and you.
also you may not know when the roof fails. Happened to me, tenants didn't notice, agents weren't inspecting and it was only observant neighbours that alerted me!
you need to go over the whole place, fix everything, get it looking really good. Doesn't guarantee it won't get trashed but at least you start from a known good state. Everything must work properly and easily.
a new law coming in soon also means you can't evict tenants if there is an unfixed problem with the property.
If Osborne has any sense (although it looks like he doesn't) tax relief for BTL mortgages may go soon.
otherwise; go read some books on the subject, take lots of advice, consider a short course in the business.
you need to oversee it all yourself. For instance, if the agent 'forgets' to protect the deposit and issue the prescribed information then YOU are responsible. You need to check that the tenants have the right to be in the UK. You need to provide smoke alarms (mandatory soon) and a CO detector. You need insurance for malicious damage, rent guarantee, legal expenses. You need to be on permanent call or at least have home emergency cover.
you can exclude some of the property from the let, tenants need to know this when they view.
expect a 3-4% gross return max before fixes.
oh, and all of mumsnet will hate you because it is an immoral business here. You'd be far better selling useless gadgets to the stupid, that is apparently ok.