There are a number of things to be aware of when dealing with parking lots, not the least of which is water runoff. That is because you have a large impervious surface that is usually designed to direct water to a catch basin or retention pond. That water is often running along an edge (curb or berm) and moving quickly. If you open up an area where water flows on the surface or against a berm or curb, there is likely to be erosion taking place as a result.
Concrete is often damaged because of an existing water situation. Make sure that is not the case here. It is more likely that the damage is from sitting water than moving water. If that is the case, you'll have to anticipate how that is going to affect your planting and how what you do there can effect what happens with that water.
Also, most municipalities have parking space requirements based on the use and/or size of the building. Removing parking spaces could make it non-compliant. Make sure you have documentation that the property owner is having you do this.
When it comes to plant selection you have to think about safety first. Make sure that you do not create blind spots where people can not see pedestrians as they drive through. If it is at a corner, you'll want low vegetation and cvan use a single trunk tree that baranches higher so that there will be very little to block the sight lines between 2'-5' in height. If you are in snow country, it is a good idea to use plants that are not easily damaged by snow plows or having snow piled on them - herbacious perennials, ornamental grasses, and flatter woody plants that are less likely to be broken by snow are best around the edge of pavement. Another thing to consider in a commercial application is to first avoid plants that tend to collect wind driven trash and not to use plants that are difficult to cllean trash out of (ie, don't use Rosa rugosa). As with any planting, it is really about selecting plants that match the soil, water, and light conditions to ensure survivability. It is always a good idea to use plants that fit in with the existing landscape whenever possible instead of having an odd planting.
Remember that it is a parking lot. Form follows function. Don't try to make it something that it isn't.