It's not a cheap hobby.
If you want to do some of the stuff yourself (backlight, prosound mod) there are some great DIY tutorials on instructables, hackaday, et al. You only need minimal understanding of electronics/soldering to do them.
As for LSDJ carts you could get a
SmartBoy Cart (Mine took a LONG time to arrive from this place) or an
EMS cart (those are the cheapest flash carts there are outside making one yourself) and both are "LSDJ ready". If you have a GBC compatible system you can at least experiment with those until you bolster up your bank account enough to get something classier.