Like some others have said, figure out the true cost to own, insure, maintain, etc. and figure out what you have to charge to do the work. Then try to get some contracts or see what the business would be like, and if it will pay for itself or just be a money pit.
If you can make the numbers work, with extra $ left over for future repairs (even if it's checked over, there's really no guarantee that an older or higher mileage engine will never break down or not have serious mechanical issues) than good for you!
If not, you might want to look into investing that money (with a broker, or a CMFP) into the stock market, so you have yourself a nice rainy day fund when the work dries out, or you have some other equipment break.
On the other hand, you can try renting a truck to see if you can make that pay. The rental costs and the cost to own might be similar.
Personally, a scissors lift might pay off better than a boom truck. At least you can get that indoors to do a high ceiling church or interior of a mall in most cases. A boom truck would be pretty difficult to manage in that case!
Don't forget, service equipment can't and shouldn't just pay for itself, it should pay for you too!