if you want nds and gba the best kit i've used and own is the m3 lite. however, 100% isn't something it has as the loader is no longer updated. i have e-mailed them several times and they assure me if an update is required they will release one.... yet it's never released =P.
a great option would be to get a good slot1 and an ez 3-in-1 as you can use rudolph's gba exp_loader to flash gba games to that kit.
look up the cyclods, acekard rpg if you want a great card and money isn't an option
look up the itouch if you want a good compatibility but money is tight
look up the r4 if you want a solid card that has no bells or whistles... i just works well which is what a flash kit should do.
i would recommend the acekard2, as its a very solid kit except for the fact that the recent manufacturing glitches have first caused DOA and now FAT errors. i ordered two 4 weeks ago and both had FAT errors. if they solve the problem get one, it is a great kit and the price is right.
you should consider other things, such as do you want sdhc support.
do you want high compatibility for download play
do you want bells and whistles like open source firmware, slow motion and save states?
a great place to start is by reading the reviews here. after a few weeks of reading and research i'm pretty sure you will know what you want.
goodluck,
-another world
