Enalapril can sometimes adversely affect renal function, so some docs want to make sure everything is ok before and during therapy and know if its the drug causing any issues not another new problem starting. I personally have never had a problem with it, and still use it even if patients have renal disease but maybe at a lower dose ( it is often used for therapy of kidney diease!) as the benefits outweigh the risks.
The following is an extract from plumbs veterinary drug handbook......
"Recent studies have demonstrated that enalapril, particularly when used
in conjunction with furosemide does improve the quality of life in dogs
with heart failure. It is not clear however, whether it has any
significant effect on survival times. It may also be of benefit in
treating the effects associated with valvular heart disease (mitral
regurgitation)"
EKG's are low cost but the blood work depending on how comprehensive the profile is could be higher cost.
In our area (NY) I would expect about approx $160
good luck with him