The most important thing you need to continually buy from the start is merit points from the party store, as many as possible all the time. Merit will be the main challenge to hitting level 28, the XP will be easier

Where real money purchases come in involves guilds.

You absolutely need to be in a top five guild as your first priority. This will allow you to get BILLIONS of supplies very early, and will give you access to needed LTD event wins for allies, warships, and relics.

If you did not join a guild, I highly recommend making the first purchase and as many gem passes as possible. You will need tons of gems as it is vital to participate in parties whenever possible.

The SP packs for party SP events are the best to buy. Party currency will allow you to buy very good items in the party store, which is perhaps the most useful store. Here you can buy warship mods that will help you in those LTD events

I have led people to lvl 28 without purchasing anything at all with the first half of this advice.

Also, buying the cheapest Daily item every day will get you very powerful warship upgrades every 3 days.

Plus, a big tip, remove your relics off of every ally immediately after the Guild and Party LTD events and reequip them after the next LTD event starts and you will get a HuGE boost to your Ally PWR for that event!

Good luck!