Not M401. The actual juice cup is very tiny and recessed. And cartridge plus atomizer is the messiest way to vape - can suddenly dump juice in your mouth. i used one the first 4 weeks I vaped and then bought more supported models with cartomizers available.
Joye 510 is fine for a light to moderate smoker of lights or ultralights who has access to electric power or USB power most of the day for charging. The batteries are 180mah so about 2 hours of moderate vaping. I only recommend the manuals - they are totally sealed on the threaded end so they are safe from being damaged or killed by juice leaks. Manuals also make it easier to get a good hit and will not turn on accidentally from wind or vibration. The Boge 3 ohm cartomizers are great on 510 for favorite every day juices instead of atomizer+cartridge (these are blank cartomizers - they would have to learn to fill them and keep them damp). A drip tip can be used on the atomizer to test juices (on manuals only)
Are either of your parents a heavy smoker, smoke full strength cigarettes, or a hard hot dragger? if so, I recommend instead at MINIMUM the SmokelessImage Volt in the 78mm size with manual batteries. This is a 3.6v mini and would be 120mm long with a cartomizer. (on manuals, you can use a DSE901 atomizer and drip tip to test juices - not good to put untested juice in cartomizers). If the person is a really hard dragger, instead consider the standard size manual kits from Vapor4Life (allows longer drags). They can use each others cartomizers and the SmokelessImage and Bloogplanet cartomizers would probably be better for them so get some of those too. (Vapor4Life cartos can be very hot on the lips and most use a drip tip as a cool mouthpiece with those, the other 2 I mentioned are more vapory and cooler on the lips)
For the very heavy smoker though it is best to skip slim ecigs but some people prefer to start on a slim one then come to the realization themselves that they need more (more charge or more heat). Models like Ego and Riva 510 from Liberty-Flights.com (Riva 510's are 3.7v there) are most popular.
Everything needs eliquid. You can get 4-7 days or more use out of one cartomizer by keeping it damp with eliquid and not singeing it. if you do not add liquid, they rarely last as long as a pack.