All cars can be modded but that is not your question, the point is whether it is worth modding or not.
If the owner feels the car needs improvement and the improvement offered for the money spent covers that need then it is worth doing.
If you have a budget to mod a car of say 2k - you'll get more power if you were starting with a 300bhp turbo than you would if you had a 75bhp nasp engine.
The power gain you get is a percentage and mods like fast road cams will usually give you 15-20% more power on pretty much anything depending on the cam profile obviously.
To some people it makes no sense to spend more on a car than the market value, and very few mods actually add to the value of the car when it comes to selling it on.
I would recommend a good popular turbo car for a project as they are easy to mod and for relatively little money you can get a lot more power from them. Specifics will depend on what you want in a car? Is it a large family car, a small hatch or cool looking coupe or perhaps you're hankering for something sporty.