Hard case lipos aren't a must at all- not unless you're racing at some kind of event or track that requires them for safety reasons, or your model's chassis leaves the batteries quite exposed- not sure what model you have so you'll have to decide that for yourself.
If you can afford it, get the highest capacity you can that will fit your model- more mAh ( capacity ) will mean the battery is physically larger so keep that in mind- measure the battery trays and check the battery dimensions, buy the largest capacity/ highest c rating your wallet allows without going crazy.
What I will say however is that look into buying lipos from reputable UK retailers if you can ( assuming you're Uk based )- hobbyking is great because they have lots of cheap options but if there is a problem then you're pretty much out of luck- most of us have played that game before. Don't overthink it though- there are literally millions of options for lipos that would be suitable, check sizes, check wallet, stick with good/known brands- there is plenty of info and reviews to read into if you want 2nd opinions, research is fun...