I think the 900 is smaller than the 830, I think it's minimatic size. Ive an 800 and and 830 - little and large - and I've had no problems at all with them, apart from when I tried to sew some upholstery fabric and it got stuck under the foot. I wouldn't worry too much about accessories, you can pick up feet on eBay for a reasonable price. Has it got an extension table? These are like hens teeth on old berninas. Somewhere I've bookmarked a link showing the history of the Bernina machines, which numbers followed which and what features were new to the machines, I'll post it when i find it.
The 125 is still sat in the basement, I keep looking at her and thinking she's worthy of a better home but I don't do anything about it. I took her to bits, cleaned her, then put her back together (I do like the smell of old machines!) but then I thought, this will just be another hobby that will get left laying half finished around the house, I'll probably list her on eBay at easter.
You'll love the 900, built like a tank and goes like a tank.