Is this for a business or professional venture? If so, inkjet would be a non starter as the colours are not waterproof so if there was any risk of the invites getting wet or damp in the delivery process then the results would not look pretty, plus, for long runs, the ink costs would be horrendous.
Laser printed media would be more stable and the output quality would be fine for short-to-medium size runs, but will limit your stock choice to perhaps around 130gsm max card, depending on what the laser printer can handle.
Depending on how many invites you are likely to produce per month, it may be more cost effective and give you many more media/stock options if you strike up a relationship with a local printing company, plus you will be able to consider more professional features like cutouts, metallic colours and coated finishes and varnishes.
If this is to be a home-brew operation then a laser is probably the simplest option, but you might want to look at a solid ink printer - like the xerox phaser range - which will give you a wider range of paper options + vivid colours + stable colours (compared to injket).
Is postscript vital for you? The current variations of the common HP-PCL language are quite versatile.
