Paying for a 3yr warranty is pretty pointless.
L-Ion do not last very long when on mains all the time. Typically they will last 18 months, but do provide a useful "inbuilt UPS" at low cost compared to a bulky external UPS and SLA battery. Laptops are useful if you have unreliable overhead power in winter.
L-Ion have a finite life, about 300-500 charges. The actual figure is determined by a chip which monitors cell voltage, if one goes too low (flattened) the battery can not be safely charged. The rumour of "cycle count chip" is not quite true, although a manufacturer may someday try it.
L-Ion require careful storage for 3-5yr life. Store at 40% charge not 100%, store in a fridge not warm ambient.
Go to Ebay, search for "genuine Acer XXXX battery". Some will be UK, others China/HK which you may want to ignore. Some will be "new", some will be "used 100% indicated", some "used 72% indicated" or "lasts 1hr 40min".
Some people buy new batteries for =A325-37 for airline trips and keep them in the fridge at 40% charge, then buy a used "1hr 20min charge" for =A312 which they use in the laptop all the other time just as an integral UPS or if transitioning to a docking station requires one to be present (power often removed).
I would not pay more than =A340 for a battery, if the cell type is listed Sanyo & Panasonic (now same firm) are good, Sony are ok, generic I would try to avoid because actual life can be somewhat less if mobile phone experience is anything to go by. Going for "genuine" means it has the makers name on it, not just identical looking.
If your keyboard is getting tatty Ebay can be good for replacements, but price can vary hugely - =A336 for months on end and then there is an glut of =A39.99-11.99 units UK keyboard & UK supplier. So it can pay to be patient.