I’m still using 1.55, because i had some issues with the 2.xx versions, the web UI doesnt run as pleasant on my RP1, the ui handling is worse and the sound get sometimes scrapped after long using times. Yeah and the webradio massacre, i try often new radio stadions and 2.xx really made it difficult.
Are there any security concerns about 1.55?
Depends if you want the new functionailty or not. Just bare in mind that although people do use the RPi1 with Volumio 2, the experience can be a little exasperating because of performance issues. The RPi with 256MB RAM is not supported (but the occasional person perseveres 
). I’m not aware of any particular security issues with Volumio 1.55 … were you thinking of anything in particular?
My suggestion is: get a pi3 and volumio2. The experience upgrade is immense from what you have now, and for 40ish euros is definetely worth