It can be as much as you want it to be, to be fair. We had a huge target to meet on my last year as secretary and it was incredibly hard work, but it was great fun and very rewarding. I have resigned because i'm woking now and just don't have the time, they have scaled things back a great deal this year.
You will need a lotteries licence to sell raffle tickets (the books of tickets not the tombola) - you get these from your local authority. Its very simple, if you go online and look on their website or simply give them a call and they tell you what you need to do. Our local council are very helpful. You are not allowed to send the tickets home in the book bags unless they are in an envelope addressed to the parents, it is worth all the envelope sticking though because this is how we sold the majority of ours. The lotteries licence lasts a few years (i can';t remember how many) and you just have to renew it each year, its not expensive.
You will need a temporary events licence to serve alcohol at events - An individual working at the event has to do this, it costs around £10 but must be applied for two weeks in advance. Again, local council, just call them and they send you the appropriate form to fill in. Generally we don't bother as who really wants to booze it up at the summer fair? People can bring their own booze to evening events such as quiz nights, so long as you aren't selling it or giving it away even (i asked!).
Anything that you do outside of the school premesis will need to be covered by public liability insurance, also if you rent out stalls to external stall holders at events. Someone up thread mentioned PTA+ magazine provides this as part of their annual membership. It is definately worth having.
Setting up from scratch will be a bit of a pain at first but it does all tend to fall together well. Keep things simple - a summer fair (although this might be too ambitious to set all of this up this year if you haven't a team of volunteers you know you can call on), maybe some cake sales (we allowed each year group to do their own cake sales and they kept half of the money to spend in their own class and the PTA kept the other half) and keep it at that, oh and a christmas fair, everyone loves a christmas fair. The sky really is the limit but in all honesty, its bloody hard work so don't make it harder than it needs to be, for me it because like a job and that was fine because i had time but it wasn't without stress.
As for setting up as a charity, that is a good idea, you'll need people to act as trustees - this is usually the chair, another member of the comittee and a member of staff, again, once its set up there is nothing else to do but having that charity number makes collecting donations much easier as many places you approach wont deal with you unless you have a charity number. You would be suprised how generous places are.
Things like licences, setting up as charity and insurance really should be down to eithe the chair or treasurer. When i was secretary i pretty much co-chaired as the chair was not computor literate but was happy to do all the donkey work so we worked well together.