With regard to keyholders we changed this year and now use Torreclean.com, who have been excellent. There are a few people offering keyholding and cleaning services on the forum but do make sure whoever you choose is registered as either a business or as self-employed.
Whoever does your non-residents' tax return should be the person who does your quarterly rentals returns as the latter has an influence on the former. If you have only just purchased your property a lot of solicitors include the first year's fiscal returns in their fees so try that company first. If not I can private message you with a couple of suggestions.
Advertising wise, spain-holiday.com and simplyowners.com are the only two I know that give you full control, where you handle the payments and have direct contact almost from the start. However a free option listing on Homeaway.co.uk and holidaylettings.co.uk is worth it while you get established as helps with filling up dates. We get a lot of repeat bookings, the vast majority of the others are from Spain-holiday (the only one we actually pay for) and we usually get two or three a year from the free listings. All the sites have lots of advice for owners and several have sample contracts that you can adapt for your own use too, so lots of info and help out there.