The budget is tight, but I have a setup that would fit the budget and leave some room for future expansion.
I would vote for the Voopoo Doric Astra.
Fundamentally, it’s a great MTL kit. It does have a built-in battery, but at this budget, I don’t think anything else is an option, because if it’s replaceable, you need a charger and all that…
But a big advantage is the 510 platform.
Another big advantage is its current price.
It’s currently on sale at A&L, and you can also get everything else on this site… coils for it… flavorings.. nicotine.
The device
https://www.aromes-et-liquides.fr/en/complete-kits/18461-voopoo-doric-astra-kit.html
Coils
https://www.aromes-et-liquides.fr/en/coils-cartomisers/15752-voopoo-ito-coils-x5-drag-q.html
Base liquid is also available cheaply in Hungary
If it doesn’t appeal to you, other options within this budget are regular Pod systems.