Still, congratulations that you managed to make all that upgrade for your laptop. I was always wondering if I could upgrade the wifi card, and change the DVD-ROM to HDD Caddy, but alas, my laptop spoilt before I get the chance. I'm interested to buy a new laptop, but it seems they've made it harder to upgrade and access the internal components with the new designs.
g55 is still a good work horse for work purposes. never failed functionally. just remember to change cmos battery when time comes (the laptop battery died a long time ago; now it's powered by ups)
right it's better to refrain yourself from getting one if you planned to upgrade parts in future. do more detailed research first about maintainability and components before buy. there are laptops that are designed for ease of upgrades and maintenance but asus seldom does that. still, it doesn't mean it's hard. one caveat: designers at asus are kind of obsessed with screws; they put screws everywhere hehe (it's good in a way; once you have an asus laptop, you basically can salvage more screws than you need for future needs of your new laptops )
for g-series, there are user serviceable and non-serviceable areas. user serviceable area usually means the area that is exposed when you remove the bottom cover. doing anything in this area as user doesn't void warranty. parts that need regular maintenance are cpu/gpu cooling systems: fans, vents, heat sinks; in the case of g55, it's a pita to maintain this part; it requires full disassembly of the laptop. at least to me, it's important that a laptop allows easy access to cooling systems. (especially those living in dusty environments) with proper maintenance, anything also can last longer.
the most important upgrade of all is still ssd as os disk. no matter what, save budget for this.