Hi hiberniia,
I would imagine that the best thing is for you to hold on to this apartment for at least a few more years. Budapest is a long term investment and if you sell now, you will probably lose money as new build prices haven't really increased in the last while. The Marina Part project is likely to be popular as location is good and there aren't that many new developments which are so close to the Danube but in my opinion, there is likely to be an oversupply of new apartments in Budapest soon and you will need to expect a lowish rent and furnish well. The fact that the apartment is ground floor and courtyard facing also decreases the rentability as I'm sure you know. Also, I think that your target market will be expats rather than just Hungarians. The average wage here is around 500 euro per month and a typical wage for professional Hungarians is around 1000 per month. Your rental income is likely to be around 400 euro+ per month but depends if the flat is bright and well furnished. This part of District XIII is being touted as a new business centre for Budapest in the next few years, but at the minute, it doesn't look very attractive and isn't really that close to the proper city centre.
Best of luck with it,
Budapest