wind trainer
I don't have a true stationary bike, but can convert one of my regular bikes to a stationary bike by using a wind trainer. The trainer is an inexpensive way to go if you already own a bike. The advantages are price & it's very light. The disadvantage is stability; you do need to pay attention & not lean way over to the side. I take spinning classes & haven't done anything in spinning that I couldn't do on my trainer. A major disadvantage to the spinning bikes besides cost, is their weight. They really are big & cumbersome. Watch quality if you go for a non-spinning bike. Tunturi is one brand to avoid.
If you have the space & resources, don't eliminate a spinning bike just because you don't want to work out of the saddle. I'd go that route if I had enough floor space (& couldn't ride outside 350 days of the year).
The standing work really works the glutes & hamstrings if your feet are clipped in. If one uses sufficient tension & proper positioning over the pedals, then the lower back & knees are protected. You didn't say why you want to skip standing work, so I'm guessing it's for safety reasons.
Debra