Over the years I have added various strobe lights and accessories to my camera collection. This has been a wonderful yet expensive journey and learning experience for me. Maybe you'll find interest and value in reading my blog(s) towards gaining some insight from my experience as a photographer towards selecting the right equipment saving one time and money in favor of spending the time out there taking pictures!
A time ago I bought myself my very first Speedlite after reading some on-line articles. I thought to myself that I just needed a basic simple speedlite for the occasional low light situations. I ended up with a decent 'used' older unit for a mere $39. The apparent cost / convenience / portability / availability of the unit appealed to me since my camera did not have a built-in flash. While this unit does works fine I soon discovered that its limitations of being manual and having limited low power/wattage. As my need for a strobe quickly evolved I advanced by buying a more current, again 'used' higher end Speedlite model with more features.
Looking back now I personally would/should have skipped buying a low end unit even though it was inexpensive unless you are seriously adverse to adding bulk or weight to your camera, have a very limited budget, you are truly only taking very occasional, limited pictures in low light conditions, or just need a flash in a pinch situation. Honestly if this is the case, I think you'd be better off buying a camera with a built-in flash or simple carrying a mobile phone with camera instead.
Overall though, I concluded that Speedlites are convenient due to their relative compact size and weight for carrying along with you, for mounting on the camera, and ease of running on batteries. In addition it is nice that there are plenty of modifiers that you can get for them. Worthy features found on many current units are TTL, E-TTL supporting automatic exposure mode and high speed sync (HHS) for using faster shutter speeds above the camera's default flash sync speed. Master/slave firing functionality will be an important feature for you if you want/need to use multiple speedlites. HHS is a real plus being able to use faster sync speeds.
Speedlites can be used off-camera mounted on light stands. Obviously this requires a further investment for stand(s), mounting hardware such as what I bought from Cowboy Studios, and optional external triggers. Many units can be fired from the camera using an integrated built-in wireless signal otherwise by using an external trigger. However this brings to mind two other considerations. Speedlites have limited flash range. If you have a need or want to use multiple lights then you need to decide whether to go with all Speedlites or not. Speedlites may not integrate easily with other strobes with their own radio system. I personally have chosen and use other strobes for off-camera lighting situations.
More to come.