I would definitely second the notion that a 10mm may be a bear to learn with. I would not worry too much about "Punch" if you are a new shooter. Worry about accuracy and control
Any service caliber (.38 Special +p up to .357 magnum in revolvers and 9mm to 10mm in autos) will do the job if you do your part as the shooter. Splitting hairs over caliber is less likely to be as important as getting a good shot and/or making follow up shots. Get good ammo. Never rely upon the gun or the caliber making up for mistakes in placement or bullet type.
A 9mm will be cheaper to shoot, though you said you will be reloading. Hopefully you have plenty of practice or the time and patience to make sure that you have everything double checked. You can hurt yourself if you do not take care in reloading. You will also likely violate the warranty of the firearm.
Besides being cheaper to shoot, the reduced recoil will make practice easier. A .22 conversion kit is nice to familiarize yourself with the weapon, but be sure to put at least a few boxes of your "carry" ammo (even if you don't carry, if you have personal defense ammo that you rely upon to protect your home/family etc, make sure you test it for functionality and reliability, perhaps as well as grouping) through your gun.
If you plan on this as a home defense gun, realize that 10mm autos will be VERY loud and likely have excessive flash indoors at night. Something else to consider is that it also has the energy to penetrate both your target as well as the outer walls of your house. Most bullets fired from service calibers will do so, but the 10 will probably leave that wall with a good deal of its energy intact.
Go rent handguns if you can. If you cannot, handle as many as you can get your hands on and find one you like. Shoot them if you can, or at least get a feel for how different calibers feel to shoot.