I don't think you would want a sampler to play back chords. If you same a chord (ie, a major chord), it will be that chord on every key you play. For every key signature you chose to write or play a song with, there are a limited number of major chords that can be played in that key. For instace, if you chose C as your key sig, you can only play the follwoing major chords: C (C, E & G), F (F, A & C) and G (G, B, and D). If you played a sampled major chord on D, it would not fit the key since D major's notes are D, F# and A - F# is not in the C scale.
Sounds like you need something with a sequencer. A sequencer is like a word processor for music. You record note information either in real-time or step-time and manipulate that musical data as you see fit. You can record one passage, rewind to the beginning (or any other place) and overdub another passage. A common way to build up a song is to fisrt record a kick and snare pattern, next overdub your hi-hat pattern. Then you can lay down a bass line and then some chords. If you have difficulty playing more than 2 notes at a time, just record those 2 notes of a chord first and overdub the other notes.
Workstation keyboards integrate a synthesizer and/or sampler with a sequencer. You have not stated a budget, but the Roland Fantom X6 is a great workstation that can be picked up used for under $1K.