Yes definitely to de-cluttering and general tidying up - even if you can't fully complete a de-clutter, 'hiding' things away in cupboards, shed. garage or even your car so it looks nice and clear is fine. This will make a big difference to the pictures which are what gets people through the door. The other jobs I would probably say no to, unless you can DIY them to a reasonable standard or get done very cheaply, if you spend more than a few hundred pounds it's generally unlikely you'll see that money back in the sales price IMO.
I would get agents round asap and see what they advise about timing and the market, November would probably be OK to put it on I would have thought but it can vary from area to area. The biggest factor in how quickly it will sell is asking price so I'd think carefully about how you balance getting the best price with speed of move, what is your absolute minimum you'd expect etc.
Choosing an agent/terms, you basically need to understand 2 things, 1 is what is included in their package and 2 what % commission will they take and are there any upfront fees. At the very basic end of the market, the online agents e.g. PurpleBricks only charge a flat fee, no commission, and will take pictures and put it up on rightmove etc. for you, but not much else (some will do the viewings too for a fee). Higher end agents will do much more by way of active marketing, printing glossy brochures, perhaps advertising in local press etc. - but they will generally charge a higher commission fee. For an 'average' house I think it's probably best to go with a middle of the line agent with a good reputation locally (either independent or chain).
To choose an agent, look at their work on rightmove, do they take good, well lit flattering pictures of the houses, do they do clear accurate floor plans, are the adverts well written, informative etc? When the agent comes round to value it does s/he seem knowledgeable about the local market, are they able to say what kind of person they'd market to, do they already have buyers on their 'books' that would be suitable/interested? Ultimately agents are sales people and it's always a bit of a lottery choosing one so for me it mainly comes down to price, but go with what feels comfortable to you...