Depends on what your going to do with it .. I have owned several tractors and for just around the house I would never buy less than a 30 hp with a loader .
TSC has a little light tester for around 5 bucks , when my daughter was little I paid her a quarter to test it for me after a while she got use to it and was no problem and do it for fun and grab other people ,, lol
try switching it in and out while taping it sometimes they stick if it works just remember to kick it in about once a week ,,,be careful and un over anyone do it in neutral
Test isn't that bad ,, Remember that they are different classification and make sure you take what you need ,, here is the only hard part you must do a hands on inspection and without proper knowledge and hands on experience with a truck your wasting your time books want teach you that . you...
What will you be using it for ? If a lot of heavy towing the diesel is the only way , if just normal towing then the added expense of a diesel is a waste
If you have to ask then never do it . We work with diesel engines daily and it can be done but not recommended . Its a process that can and will destroy a engine if not done correctly