Steps to make my 5 year old Macbook Air (mid 2011 13" model) like new again

I bought the above in Sept 2011.  I have never formatted etc like I would with an old Windows based machine to keep it clear of DLL files etc.

The macbook has become quite slow (mainly because I have like 70 chome tabs open) plus have used up lots of space (10gb HD / SDD space available out of the 120gb).  Apart from some grubby finger prints and a battery that is wearing down (condition reports as: replace soon), the laptop cosmetically looks new.  So my questions are:

1. Replacing the battery - I used to build computers as a hobby so I guess replacing this battery should not to bee too much effort right?  What model do I need and where is it best to get it from (UK web sites pls) if I want to avoid paying Apply high sky charges.  Does replacing the battery myself create any issues with Apple even though it is not in warranty

2. If I want to format it or reset it to factory settings, what is the best way to do this?  If I use time machine with an external hard drive, do I then not need to worry about moving any files off the macbook or doing other forms of backup?

3. Is there not an equivalent of disk defragmentation for Mac - I have never done this or found a way to do it compared to my windows machine....I think I was told that the mac manages itself pretty well....but still, is there no way to force to defrag?

4. I'm running the latest version of El Captain - If I do reset it to factory reset, is the install software now built into the mac or do I need to connect a CD drive to it and get the software from somewhere?

5. My charging cable is in tatters for the last 2 years so much so that the metal braiding has shreaded under the first plastic covering, to now reveal the final inner white covering.  Is this dangerous?  I have tried wrapped it in insulation tape in some 10 places on the cable but maybe time to get a new one?  If so, avoid non genuine/originals or are there reasonable cost quality alternatives?

Finally, is a 5 year old mac that look good still, still regarded as worth keeping?  Or have they become very old technology now, esp since I cannot upgrade the 4gig ram.

Thanks.