It will be the only one you need and you won't need to spend again in the future (resale in the aluminium tanks is terrible so you are unlikely to be able to sell it to put towards a fibre wrapped)
This is the same advice I used to read every where for new players 10 years ago
Unless you stick to one setup or buy and sell many players end up with multiples of everything
On the scale of things in paintball a small payment of £30 on an alumimium is not a waste
On the contrary £30 for a 5 year cylinder if you never retest (or £30 for a 10 year cylinder if sites recognise that alumimium are legal for 10 years)
That's £6 or £3 a year, and around or under the price of a case of paint
A fibre at £150 commits you to invest in retests or to sell as second hand
Retest and you're looking at around £25 every 5 years, don't and you've shed half the cost on your second hand sale value
That's £200 over 15 years, £13 a year
Start with a basic alumimium now and your 'worthless' cylinder when you upgrade to fibre is your spare
Extra capacity to refill for 'home testing', a loan set when you bring a mate, an emergency spare if you blow your burst disk or when waiting for retest, or a good deed give away to a new player