Some conversion kits may work, some won't.
If you are going to rely on the handgun to protect your rear end, do you really want to buy something inexpensive?
In days of old, it was not uncommon for people to pay 1 month's salary for a handgun. Even in today's wages, that still will get you a quality handgun.
If you want a .40S&W caliber handgun, then buy one. There are quite a number available in the $400-$500 range. Especially if you look at the used market. Police turn-ins are normally carried a lot, shot a little. If you could get by with a couple of external scratches....
"A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."
Life Member NRA
Life Member NAHC