(Regrettable – sorry just not a Christmas fan) Season’s Greetings All!
 
Apologies in advance for any novice or downright stupid questions or pointless information below, but despite doing as much prior research as possible, this is the first time I’ve ever had to change a car battery. It does seem to be quite the endless minefield though. I also apologise for the length of this post, but have tried to provide as much information in order get the most definitive answers possible and possibly provide similar less experienced owners in the same situation as myself with the right advice that I’ve found hard to come by.
 
Requiring no fewer than four jump-starts for my 2001 MK3 in the last year, has led me to believe I require a new battery. The two I needed earlier in the year were the result of carelessly leaving the internal light on overnight, the bulb of which I then removed to prevent this mistake a third time. The two most recent flats, however, have both come within the space of the last fortnight, due to no fault of my own. One occurred after a particularly cold night despite driving the evening before. Last night the battery was flat again, despite the weather being rather mild this week, seemingly because I simply hadn’t driven it for four days.
 
While some might simply indicate these are obvious causes, my current circumstances dictate my MK3may go undriven for several days at a time, nor can I obviously control the weather. Either way, while it may be fine to keep things ticking over for a typical driver who does several miles every day without fail, the battery I currently have simply isn’t fit for my purposes. Moreover, despite lying undriven for over 3 weeks in the late spring(whilst I was in Japan, fittingly enough),it started fine first time, leading me to believe the condition of the battery has rapidly deteriorated in recent months.
 
I’ve also taken into account appropriate factors and occurrences which indicate it is the very much the battery at fault and not the alternator or starter motor. With research, the gods of internet consensus very much point in the direction of a new battery. This, however, is seemingly where this consensus ends and I therefore require some advice on selecting a new one.
 
Every site I’ve been to has been trying to sell me a different battery of a different size. Despite my current battery being a 063, I’ve been recommended various types, from a 005L (which I’ve read is the official correct size) to a 048 and a HB057 (also known as 049, I believe). Basically anything and everything other and far larger(at least in height) than the 063 I currently have.
 
My questions if you’d be kind enough to answer are therefore:
 
1) Do you agree I am in need of a new battery?
 
2) Does the actual battery type/code/part number particularly matter as long as it physically fits, or could my situation be the result of my current battery being somewhat unsuitable?
 
132133) Being a Japanese car in Europe, I’ve read there can be issues with battery terminal size, or is this only an issue with actual imports?My current battery is certainly instandard configuration, with the positive on the right, but is the size of the terminal nodes something I need to keep an eye on?
 
4) Should I definitely buy new, or could I save some cash buying a standard or possibly even a higher capacity used, but not too old battery, in perfectly-decent condition?
 
148149) Being an old car I don’t know how long I will keep, and currently jobless, I’m keen to spend as little as possible, within reason. For example,I could replace like for like (new) for around £30 to get a type 063 35/39 Ah, 330/360A, compared to the 41Ah 360/390A, I currently have sitting flat. Would this do the job, or would you recommend investing in something with a higher capacity? I realise greater Ah will hold more charge and CCA aid with cold mornings, but will this really make any noticeable difference as opposed to simply getting anything that’s new? Given I’m currently only doing short urban journeys (max 3 miles/day, 6 days/week) would a larger capacity battery even have the chance to get properly charged?
 
 
Many thanks for reading. I look forward to hearing your thoughts.
 
(Christmas) Cheers,
David.

Once a battery drops to 5ish volts or below, its basically cactus. I imagine the first instance of leaving your lights on and flattening the battery has started the downhill spiral. I had the same problem.

Your only necessity when choosing a new battery are that the CCA are the same or above factory specs. Any battery shop should be able to supply you a factory battery with a warranty.

You're wasting your cash buying a used battery IMO. If you take good care of it, a new battery should last a very long time. If you're concerned about not getting the battery charged enough in your daily commute, maybe invest in a trickle charger.

IMO, for a street driven car, always buy the highest CCA battery you can afford that will fit in.
 
An automatic or 'smart' trickle charger is a great investment for a car that is only used occasionally, and your battery will last longer if it is kept fully charged. 
When you get home after a drive, just pop the lid and connect the trickle charger to the battery terminals.
Even if you don't drive it again for weeks, your battery should still be fully charged.
Check out:
https://www.batterystuff.com/kb/articles/charging-articles/how-do-i-pick-a-battery-charger.html