Team building help

What are your CEs?

there are many low stare servant really usefull (even more in camelot)
Cu, Euryale, Hans, Cesar, Robin, Lu bu, leonidas, shakespeare, arash ushiwakamaru and a few other
are all good servant, and they even compare with some 5*
and mash is going to be extreamly strong once you have completed the sixth singularity.
but if you want a fast and easy way to have more success...

Level up mash leonidas and georgios.
Invest in hercules skill (at least to level 6)
and do a "herc will survive team" with lot of def up, tount and all in all, defend herc.
I actually completed camelot with mash and cu alter (who is similar to herc in playstile)
if you want to know who should you invest into... as anyone else you should hope luck bless you whit merlin, simply becouse if you have him you will be able to use any damage dealer in the friend list, and all your problem would be solved.
Still, roll for love.

Depending on what boss you're stuck at in Camelot, yes, leveling Euryale would be very useful (and she's a good anti-male ST Archer in general). Also, with Babylonia in mind, make sure you have at least some Balanced or Evil Servants leveled up (or available from your friend list).
Also, what exactly do you mean by "complete the game"? The main story is still ongoing on the Japanese server, and it's two years ahead of the NA one. Main quests are relatively easy enough that you should have no problems with them if you have a good AOE and ST Servant of each of the 7 main classes (like other people said, they don't have to be of high rarity, there are very good 3-star Servants).

Edit: forgot that you have Heracles, then Balanced/Evil Servants aren't that necessary, he'll do fine. And yeah, if you like Merlin, try to roll for him when he's available, he's really OP.

Level your poor kouhai, Mash is great utility, especially with your herc, can buff his defense, draw agro for a turn and make him invincible for a turn, so he gets 2 turns without damage unless the enemy has AOE attacks.

As for 3 stars to raise.
Ushi and F/SN Cu are both 3 stars that I still use even though I've got 5 stars as they have very good niche's, Ushi at NP5 hits really hard and Cu is great for long fights as he's really good at living.

Hans, Mozart and Shakespeare are all low rarity, high utility servants with great skills.

As a F2P, the only 4* support(ish) servants I got were from my initial free pull (got lucky, or at least I thought, and got them plus Carmilla--but not Herc, and got tired of rerolling after 5-6 times...). And for the longest time, Mash was really my only support. But then I *finally* got Hans (took about 2 months of FP gacha to do it, and then at least another month or two to NP5 him). He really, really made my teams stronger. With 3 Arts cards, he can indirectly help build NP charge (obviously not like a Waver, though), and 3,000 points of heals (or more if you can overcharge him) that he gives to each servant, usually at least twice per fight can really make a big difference. He carried me through both Camelot and Babylon (I think that at most, I used a support Waver twice, only because I wanted to NP on the first round and the best charge CE I have can only get me 60%).

So I second the opinion someone else stated: level Hans and use him.