Most attorneys that handle evictions on a regular basis have a flat fee for service. To my knowledge none of my clients have ever used an "eviction service" so a cannot speak to that. I would simply find a local attorney who specializes in eviction.

You may want to offer them "cash for keys"
This should be a written contract that spells out when they must leave(usally 2 weeks), the condition the property must be left in, and the amount they will recieve to leave peacefully. $500 is what most lendors will offer in my area. If you approach the former owner with compassion and understanding and create a small win for them you might save the hassle and expense of eviction plus the cost of a clean out.

It depends on the situation, please keep in mind that you are dealing with someone that has just lost their home and this can become a dangerous situation. If you have a a disgruntled previous owner you may want to enlist the services of a real estate lawyer. If you feel that this is a peaceful situation then a real estate agent will suffice. Either way, evicting someone is a legal process and must be approached the same way whether it be an agent or a lawyer.