So A bit of a long read but I was in a Similar situation to you around a Similar age, I was 22 in 2017 when I was driving a 1.9 tdi Sportline, Been a Car enthusiast always have an itch for another car.
Drove the TDI for awhile but always had my eye set on an E90 330D, in 2018 June found a great spec LeMans blue 330D which I bought while I still had the TDI, after 2 months sold the TDI as it was just standing as I was dailying the 330D on my 140km round trip commute to work.
Ended up spending quite a bit of money on the 330D modding it, including Software and A full straight pipe setup, people under estimate how good the Diesel straight 6 sounds with a straight pipe, most people thought it was a 335 until they saw the Badges.
Fuel Consumption wise, If you drove decently you got great fuel economy but if you got your foot flat most of the time you payed the price.
In February 2019 The itch came back as I found a Beaut of a E92 335i N54 Manual, every mod was done the only next step was upgrading the turbos, pulled the trigger while still having the 330D. Two Months later I sold the 330D as financially it made more sense for me to get a smaller car for my commute and keep the 335 as a toy because boy oh boy was she thirsty there was no way I was going to be daily driving her, 450km on a full 60L tank of fuel while featuring the throttle. Ended up replacing the 330D with another 1.9 TDI Sportline as my daily.
I owned the 335i for exactly 2 years sold it in February 2021 as I needed to purchase a house, As much as I would love to have kept it financially did not make any sense but as you get older you realise cars come and go.
The 335i was an amazing car, its an experience like no other however that experience costs a lot, In my two years of Ownership only thing that had to be replaced was the waterpump which cost 8k however my car I knew had been looked after and meticulously cared for by the previous owner. the amount of maintenance parts on a 335i that needs to be taken care of can add up real quick, just a few as an example: waterpump, leaking injectors, High pressure fuel pump, leaky boost pipes, valve cover leak, carbon build up on intakes, vanos solenoids, Turbo failure, wastegate rattle, etc. Those are just things that are common issues which were more towards the N54 which I had but some still carried over to the N55 as well.
If I looks back at it now I wish I could have bought my house and kept the 335i but life happens, I still got my second 1.9 tdi as my daily and in the future will get another car which excites me.
Things to think about:
Definitely keep the 330D as an option, maybe test drive one and see,
@JdW is 100% correct the N57 motors are bullet proof you literally just keep up to date with your Services and check the vibration dampener pulley every 60-80k km for any cracks change if needed and carry on enjoying great fuel economy and power when needed. Its the best feeling seeing the face of the guy in his GTI getting Gapped by a diesel.
If your commute is only 12-15km then the 335i can be used as your daily however they are performance vehicles and are prone to problems, you have to see if the car needs repairs will you be able to sit without it for sometime. If you do plan to daily would recommend finding one that still in motorplan or definitely take an aftermarket warranty. If you decide to buy one which is out of plan and don't take an aftermarket warranty make sure to find a mechanic or workshop which you can trust wont screw you over as most of them will end up costing you triple just changing unnecessary parts trying to figure out what the issue is as troubleshooting some issues on 335's can be a nightmare.
Almost everything for a BMW is expensive at the Agents but if you are handy or have a good mech things get get done way cheaper without compromising on quality, for example the 330D needed new Pads and Discs front and back, agents quoted 16K, bought the Pads and Discs from incomex in centurion which costed R4300 and fitted myself.
The price of a service for your i20 is going to be a 3rd of the cost for a 335, Tyres will probably be 4 to 5 times the cost compared to your i20 depending if you continue with run flats or not, In general anything that needs to be replaced will cost at minimum an average of at least double what your i20 would cost.
Depending if you go manual or auto, if you go Manual the clutch would probably cost 3 times the price of your i20 clutch, and if you go auto if the clutches in the box start slipping an overhaul at ZF (they make the gearboxes in most BMWs) will cost between 35-45k while a replacement you looking around 70k. Also Auto gearboxes BMW say are sealed for life which is not true as they dont care what happens after motorplan, the oil in your auto box should be change every 80k KM, ZF do this.
Its always great having two cars but sit down and work about your budget and every cost involved for both cars such as, insurance, services, maintenance, unexpected repairs, yearly licenses, etc and make a decision from, remember the most important thing is you don't ever want to over commit and become car poor because your whole salary is literally paying for your cars.
If you do decide to go for a 335 make sure the one you looking at has been taken care of, get the VIN and check with BMW if there are any comments on the vehicle, this will tell you whether it was in an accident while it was under plan, get a print out of the motorplan, this will tell you every part which was changed under plan, join the BMW Fanatics forum, pretty active and plenty of knowledgeable people there and people who can help you check all this info with a VIN.
Last thing to remember is, you only live once, do whats going to make you happy at the end of the day.
I hope all this information helps you make an informed decision, feel free to PM if you want to ask about anything specific.
