If you beat Camelot already you shouldn’t have much difficulty with Babylonia. The difficulty is about the same, but with less gimmicks and maybe beefier mobs. There’s only one significantly challenging (and rather gimmicky, incidentally) fight in Babylonia.
Notable low rarity servants that most people use are Hans (stars and healing support), Georgios and Leonidas (taunting), Arash (farming, one-time nuke), Spartacus (class-agnostic farmer with battery), David (party evade) and Euryale (premiere anti-male Saber/Zerker specialist). A little more niche are Mozart (plug-in 50 crit stars for one turn and Arts buff) and Shakespeare (plug-in Buster buff).
Notable DPSes that hit extremely hard for their rarity are Bedivere (strongest Saber NP unless you have multiple NP levels on gold servants), Ushiwakamaru, Robin Hood, Caesar (supplants Bedivere in two years, often used as a buff bot). Cu (FSN) is notable for his extreme durability thanks to his spammable evades; he’s often used as an anchor, though his damage is just moderate. Lu Bu has an extremely hard-hitting NP, but has difficulty gaining NP and is usually a one-shot due to his lack of survivability. Jaguar Warrior is actually low-key very strong, but suffers like Bedivere in being story-locked, so you have to invest through specific gacha banners to get her up to higher NP levels.
There are other interesting low rarity servants, but those are the main ones. The others tend to be stopgap solutions until you pull a gold servant, or pretty niche, like Asterios in certain boss fights and Inshun for soloing. Paracelsus will eventually be a key piece for looping setups.