What is the differences between 818TQ+1000 and 818TQ+-100LP chassis, I will use an Low profile add-on card, does this mean I can use either one?
No, this depends on the Quad AMD mainboard that you are going to use.
The 818TQ+-100LP chassis can support 32 Dimm Quad AMD boards like H8QM3-2+ / H8QMI-2+ and has only an LP slot, die 818TQ+1000 supports only Quad AMD mainboards up to 16 dimm boards like H8QM8-2+ / H8QME-2+ and has support for FH/FL (full height / full length) slot.