I fretted about the CIC thing for some time - I am satisfied (following various conversations with HMRC - and my accountant) that in all but the most obviously exceptional cases, it's a bit of a red herring. Most people will be unlikely to fall foul of CIC unless they either purposefully set about to exploit the system, or they don't keep an eye on the balance between investment profit and profit from regular trading.
When do you become a CIC? HMRC states that your company has to exist 'wholly or mainly' for the purpose of its stated trading activity. 'Mainly' is the key here - and HMRC will exercise fair judgement. eg If 95% of your profits come from regular trading activity and 5% from trading, no problem. If the amount from regular trading stops being the 'main' element, you could be in trouble. Neither they nor I seem to know where this line is. I suspect the fogginess is more to deter those who have set out to exploit the system rather than catch-out bona fide traders who happened to make a bit more than expected from investments on the side one year or another.
In terms of the benefits of LC vs sole: personally LC suits me more. For me, the ability to retain profits is one of the most useful perks. Were it not for retained profits, I would be paying a stack of cash at 45% on the additional rate. In reality, we (myself and t'other half) need not stray out of the basic rate for all our needs.
£400 should be sufficient for an accountant to handle the LC return, assuming your book-keeping is fairly in order.
PAYE can be done for free using HMRC's 'basic PAYE tool' (bit of a pain to set up, but HMRC can talk you through it on the phone and once you've done one, your monthly PAYE will take 1 minute,
Self Assessment can also be done for free (as it sounds like you've already managed) - dare I say it, HMRC have actually made something easy in this respect!
Finally - congratulations on your business!