Well, first of all there are more or less three major train station in Budapest (Keleti - most of the international trains depart from here, Deli - trains to Croatia/Slovenia depart from here & Nyugati), so you should decide where exactly you want to stay. At the same time all the stations can be reached easily by subway, so in general it doesn't really matter where in the city center your hotel is located, at least as far as it's not far from subway, then you can be sure that you get to the any train station in less than 20-30 min.
Regarding hotel choice & hotel reviews we usually recommend (and use ourselves)
hotelscombined.com, it analyses data from various hotel websites and allows you to get the best deal for less, plus it shows all the hotels on the map, so it will make you search even easier
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