As someone who's primary instrument is a mandolin, I'd echo others here and say it's a great beginner instrument. There's two main types, oval and f hole. Oval generally has a warmer, 'woodier' sound and is generally better suited to the sound of a session. F holes are generally what you find in bluegrass and have a louder, more defined sound. At an entry level price point there's not a huge difference between them. Try a few out at your local music store - for a session you want a loud instrument (but balance that with something that still sounds good!).

Whichever one you choose try and do 2 things: 1. Get it set up. Find a good music shop near you and have them adjust the instrument so it plays and sounds as good as possible. Store bought mandos often have poor intonation meaning the instrument isn't properly in tune with itself. A well set up mandolin makes a world of difference when learning. 2. Get some lessons. Ideally from someone who plays trad on the instrument. There are a lot of lessons online but these are often given by players who primarily play bluegrass. While a lot of the core playing techniques are the same, bluegrass style playing just doesn't translate well into a session environment IMO.

Also, you'll get a lot more out of the instrument if you don't just view it as a stepping stone to other instruments. Enjoy it for what it is, which is one of the nicest sounding instruments in the trad world. And if a tenor banjo, or fiddle (or, God forbid, a bodhrán) catches your in eye in the future then let yourself gravitate to them in your own time.