Well after a few quick moments of research on both the AEGs you enquired about I was able to come up with a few suggestions.
For the G&G GR16 R5, your options are pretty open, as it stands it is a rewired AEG with full stock so you should be able to put pretty much anything want in there in terms of battery. A few suggestions would an 8.4 mini battery comprised of 2/3A cells or a 9.6 volt battery comprised of Sub C cells. I personally would recommend going for an a 9.6 or even an 8.4 V comprised of Sub C cells. In regards to how many Mah the battery should be that is entirely up to you as it stands more man means longer run time.
As for the G&G GR16 R4, from the majority of the images I have seen it looks as if the battery is housed in a fake magazine on the side of the AEG in a ready mag of some form. Unfortunetly I do not know what to suggest, It looks as if an 8.4 v mini would have no issue fitting in the fake magazine, even a 9.6 v battery made of 2/3 A cells could fit. Essentially I believe your restricted to whatever you can fit into that mag.
has an airsoft section where you can select and purchase pre designed packs which will be assembled and shipped to you as soon as you order. Also it is possible to have them create a pack specifically for your needs. Just remeber to keep in mind the kind of connector you have on your AEG in regards to which connector you select for your battery pack. Match Male to female, so your AEG will have a male end, just make sure you select the appropriate matching female end.
As for a good quality charger charger here are a few listings.
This is a good quality reliable affordable charger:
If it is with in your budget I recommend something like this:
Its recommend that you get a good quality charger that has the ability to select the charge rate, and auto disconnect or shut off when charging is complete and its also nice if it has a display screen so you can read the output, but perks like these do cost extra however they are handy. I recommend avoiding simple wall chargers. The ones that you simply plug into the battery and leave. They have a set charge rate and no fuses to provide any form of protection, and if you forget about your battery (god forgive anyone that does) you may accidentally damage your battery by over charging it, or worse, you could set your house on fire, maybe, probly not but you never know.
Best of luck.