It was some years ago that I appealed my assessment in Allegheny County, but I chose to do it myself.
In Allegheny County all the information needed for my appeal was available on the county web site, including the assessments of every other property near us as well as the ratings of our property they used for the assessment and will use during the appeal proccess/hearing. Condition rating of property, breakdown of the rated value betweeen lot and structure, dimensions, as well as the properties they used as "comparables" in rating our property.
I took pictures of our property that made my case and brought copies to the hearing. In our case there was a power line right of way between us and our neighbor with a tower on it, which might scare off some buyers. Bathrooms that needed remodeled, anything that would come up as flags or potential negatives to a prospective buyer.
The appeal "judge" reduced our assessment somewhere between the value the county had revised us to and where we were previously. I believe it better to be reasonable for what you are asking for in your appeal.
Another thing working in our favor was that our real estate agent advised us to "chatel" some items when we bought the house. That involves picking our upgrades that the seller could have removed (fireplace front, bar and stools, area carpets, light fixtures, etc. as well as anything outside they could have taken) and listing those items on the sales agreement and then breaking down what we paid between the actual property and the total of those items. We were conservative and noted that about 10% of our purchase price was for those items and 90% was for the property. So the recorded price with the county was the 90% number, which was still the latest selling price on our property and worked in our favor. Obviously you can't go back in time to do this but if you buy again keep this in mind.
But if your would not be comfortable doing that and the price of the attorney is reasonable you might be better off with the attorney. It seemed that the attorneys that want our business for this type of issue get a good bit of this business and they, the county in our case. schedule their cases back to back so the attorneys really don't have to dedicate extensive time and can therefore make it somewhat reasonable. Is that the case?