Speaking in general there are two possible issues when playing compositions written for 88 keys on a keyboard that has only 61 keys:
(1) The left-hand and right-hand parts might be further apart than is possible with only 61 keys. However, keyboards often let you "split" the keys into a left-hand zone and a right-hand zone, with the ability to set the voice, octave, and other parameters for each zone. Thus, you could split the keyboard, assign the same piano voice to each side of the split, but use different octave shifts for each side, such that the keys in the left-hand and right-hand zones are further apart from each other in note value than they physically are.
(2) Some compositions require you to play back and forth across the keyboard, such as starting down in the lower portion of the bass clef and then playing your way up into the upper portion of the treble clef. In this respect, note that as few as 49 keys are enough to play two octaves below Middle C and two octaves above, which covers all of the notes written on the lines of the bass and treble clefs, plus a few notes that go beyond and must be written with one or two "ledger lines." So that's one of the things you need to look at in the compositions you're interested in-- as far as the notes written with ledger lines, how many ledger lines do they use; and is there any notation to indicate that you're to play the notes which are an octave higher or lower than written? In any case, most keyboards let you shift the keys up or down by one or more octaves, so if you need to play a sequence of notes which spans more than 5 octaves then you could shift the keys up or down an octave or two as needed.
It might also be necessary to combine these two techniques, as well as modify their settings while playing. For instance, you might need to split the keyboard into left-hand and right-hand sides that have different amounts of octave shift applied to them, then change the amounts of octave shift as you play-- or even change the location of the split point, such as switching back and forth between two octaves on the left and three on the right, and three octaves on the left and two on the right, or something like that. Often you can set up a keyboard in a particular way as far as the voices chosen and all the other settings, and save the setup as a "registration." Then you can recall whichever registration contains the specific setup that you want to use. The keyboard may or may not have multiple buttons which are dedicated to recalling specific registrations with one press of a button, and as you can imagine it's better to have more of such buttons than fewer of them.
The EW400 lets you save 32 registrations, which are organized as 8 "banks" of 4 registrations. It has 4 buttons that let you recall any of the registrations in a given bank with one press of a button. In contrast, the EW300 lets you save only 9 registrations, and there are no buttons dedicated to recalling specific registrations with one press of a button, because you must select the registration you want to recall using the numeric keypad. So as far as using registrations while you're playing to overcome the limitations of having only 61 keys, the EW400 would be a better choice than the EW300.
However, keep in mind that splitting the keyboard, shifting the keys up or down an octave or more, and recalling registrations as you play require a different style of playing than if you had 88 keys. In other words, if you learn how to play a particular composition on a keyboard that has 88 keys, then try to play it using one or more of these techniques on a keyboard that has fewer keys, you might almost need to learn how to play the song all over again because of having to retrain your brain as to how far apart to spread your hands, having to reach for a button to recall a different registration in the midst of playing, and so forth. Likewise, if you learn how to play a composition on a keyboard that requires using one or more of these techniques, then you try to play it on a keyboard that has 88 keys, you might find yourself stumbling because of having to spread your hands further apart or not having to punch a button to recall a registration, etc. I'm not suggesting that you won't be able to make the switch from 61 keys to 88 keys, or vice versa; but you might want to consider how easily you're able to adapt to changes. And if you can, do look over the sheet music for the compositions you're most interested in, so you can determine how many octaves they're written for.
The other major consideration has to do with the types of keys-- both the shape (box or diving board) and the action (unweighted, semi-weighted, or graded weighting). If you're primarily interested in piano compositions, you might be best off with a digital piano rather than an all-purpose arranger-- or possibly an 88-key MIDI controller, assuming you don't mind having to play virtual instruments on a computer, laptop, or tablet.
ADDENDUM-- I just realized that I keep referring to the EW300 and EW400, which have 76 keys, whereas I probably should have referred to the E363 and E453. But the discussion could still apply to anything less than 88 keys.