2001 Pontiac Bonneville Preventative Maintenance

I've had this car since 2003 when it had 70,000 miles on it. It now has 435,000. I've used synthetic oil in it and changed the transmission fluid/filter twice. Car still runs perfect with NO engine or transmission work. I plan on driving this car on long trips and want to minimize the possibility of a major breakdown while on the road. Is it a good idea to replace the timing chain/gears/oil pump/water pump now as preventative maintenance? My mechanic said there is a good amount of slack in the chain. He had a socket on the harmonic balancer on the crank and moved it back and forth. I bought aftermarket parts for all of the above described for $160.00 Mecanic's labor estimate is $250.00 I also have considered doing the work myself and wonder how difficult it is to do. The last timing chain I did was on a 1972 Plymouth and a 1963 Mercury with lots of room to work as the engines were mounted differently, not sideways as they are now. Your advice is appreciated as would be a site or link to step by step instructions to do the job. I'm thinking for $250.00, let a professional do it. This includes installing an oil pump repair kit. Thank you. Mike