Bar end shifters are very, very easy to use, even in friction mode - and friction mode gets easier the lower the number of cassette sprockets. Therfore, they are a perfect sollution for your outdated 7 speed system. They are a great shifting option, especially for odd-spaced cassettes that don't lend themselves to modern integrated shift/brake levers. And for some, they are considered better anyway because they are so dead simple mechanically, lightweight, cheap and easy to use.

I would absolutely go with friction bar end shifters if that were my bike. Any bar end shifter will do, even indexed, as long as it has a friction setting.

I would never go with stem shifters (I just hate them!), and while I do fine with downtube shifters, I don't recommend them to anyone who doesn't really, well, love downtube shifters. They really don't hold any advantage to bar end shifters in practical use, and have the disadvantage of having to ride one handed and reach down to feel for the shifter and then feel for the correct adjustment. Same with stem shifters, but an even more awkward position, imho.