Ctek MX 5.0 has solved all my car battery woes. It really does work magic.
I have the C-Tek MXS 5.0. Appears to work good but runs worryingly hot
I got it for about $150, I got it so I could leave my car on trickle charge when I went overseas (Car had a weak battery at the time, would go flat after 2 weeks or so of non-use).
In OP's position I don't think I would get a charger. If tight on money I would get a cheap ($30) set of jumper leads to go in the car, and use the battery until it fails to start , then beg a neighbor/ workmate / passer by for a jump, then replace the battery with a warehouse/super cheap one or similar.
If not tight on money I would either just replace the battery, or go to a mechanic / auto electrician and get them to test it and replace if needed (issue might be something other than the battery)