Is there maybe an agency I could use to purchase the Warsaw-hungary SparDay or SparNight?
Well, we aren't really big fans of advertising/recommending agencies, since in most cases they charge rather high commission and in addition quite possibly might fail to provide on time the thing you need etc. Plus in your case actual commission might be higher than difference between Spar ticket and return PKP-MAV, thus, imho, not worth powder and shot.
Maybe you have a friend or somebody in Poland who can help you out with it? If not, we emailed you link for the most known polish agency via private message, but be careful not to overpay
... please tell me the price of regular one way fare from Warsaw to Budapest.
Since there is no discount for one-way ticket, the fare is almost insane for Central Europe - almost 93 EUR, thus we definitely not recommend going with such option.
Doing tricks with ticket doesn't really seem worth it, getting off and on the train would mean braking up the journey quite often and possibly lenghten it to much.
Actually time loss might be not that essential or even not existing at all, plus you won't need to change trains till the morning or at all. For instance, purchase Warsawa-Bohumin RT ticket (37 EUR per person), board Warsawa-Budapest carriage, it stops in Czech station Bohumin for almost an hour waiting for reattachments, during this time you simply visit ticket counter and purchase two separate tickets: a)Bohumin-Sturovo RT CityStar for 2 (will cost 68 EUR in total) and b) Sturovo-Budapest RT (14 EUR per person or even cheaper if you <26). The second ticket can be also purchased in Poland for the same fare. With such ticket combination you cover the whole route and won't need to get off train at all. Such not very complicated trick with buying tickets in Bohumin will give you total fare Warsawa-Budapest-Warsawa (for 2 passenger) app. 170 EUR, instead of 2*112 EUR with the direct tickets.
Also, you can save even more, but then will need to get off train in Sturovo, walk over the bridge to Hungarian Esztergom and arrive to Budapest +1 or +2 hours later then Warsaw-Budapest carriages, but that's might be already to much of a hassle though.
What about RailPlus though? From what I've been told it offers 25% discount on international train fares.
Yes, RailPlus card will give you -25% from the original ticket fare. But you first need to have that RailPlus card. It costs money and purchasing it (price depends on your age and country you purchase it in) might again be not worth powder and shot, unless you plan to use it few times, thus it can pay off.