A_N_other_beginner

I've got a Celestron C6N that I really need to get to grips with. I appreciate it's not the best time of year necessarily for Saturn/Jupiter, but I had a glimpse of Saturn's rings the other night and would really like to be able to show as much as I can to my eldest, the interest is clearly there. Firstly - maintenance issue. I had Jupiter centred last night, and when well out of focus (but with the light of Jupiter to make the mirror visible through the EP), the donut shaped light looked like a science textbook of bacteria on a petri dish. Is that normal or a sign of dirt/something that needs rectifying? It's been a few months since I used the scope and when I did I had frosting issues, my concern is if this is a sign of mould. Secondly: my EP collection is limited and needs to grow. I've had a look at Jupiter with a mix of 10mm BCO and a cheap Vixen 2X barlow that I mainly purchased for attaching my DSLR for lunar photography - so not ideal. But 150x, which I gather is in the right ballpark for Jupiter? All I could make out was the faintest of banding. I appreciate the needs may be different for Jupiter vs. Saturn, but I'm considering either a better barlow (Televue 2x?) or something like a Vixen SLV 5mm or push it up to 187x with a 4mm. I've got a budget of about £100, where would I gain the greatest benefit - either of those options, or something else?