typically you'd want to replace all the engine mounts 1 go since labor cost/time required etc. and you probably have no idea on the condition of the existing engine mounts.

consider replacing the ATF (or manual gear oil, cant remember your car is AT or MT?) and if AT, replace the filter too (basically doing full flush/drain already) and its gaskets the next time you're replacing it since you've mentioned that you already did it.
consider performing engine flush too, and throttle body/carburetor cleaning as well.
consider spark plug cable, spark plugs, and even distributor rotor replacements too after inspecting their condition.
get a voltmeter/other utility to check on your car grounding current, if low, do clean your existing grounding cable from oxidization. if still low, consider adding grounding cables. recommended ones are ericteoh from LYN. please do NOT ever consider cheap HKS imitations and other similar grounding cable products, they are utterly useless.

for rusted parts, if the rust has gone deep into the metal, there is no choice but to replace them as it is already penetrated deep and coating above is only to hide the problem. eventually rust oxidization/byproduct will surface and will push away your aluminum coat. (similar to how "bubbles" form under even fresh sprayed paints for really old vehicles, like the roof of my car )

as for the exhaust, consider spending rm200-500+ for a good s-flow muffler (and even a good mid bullet if you got additional budget to spare) that will help suppress most of the noise, while maintaining performance
if you're not looking for performance, even chicken brand will suffice

and yeap, good choice on the car and yes it is really cheap compared to any other choices

some tip on washing (if you're ever going to learn detailing like i am picking it up now):
buy proper car wash product (please do not use dishwashing/laundry soap/detergent/product ), and consider claying > polish > wax if the previous owner neglected the paint/car exterior

for the budget concious, i highly recommend kura kura [lol turtle] wax F21 car wash
http://www.turtlewax.com/detail-F21%20Car%20Wash-9-111.aspx

http://www.turtlewax.com/detail-Zip%20Wax%...064oz-9-43.aspx
zip wax + car wash if you're either lazy and/or you don't want to bother with detailing

oh reminding me to get a camshaft oil seal as well, thanks lol

1) usually between 70k-100k km (hence timing belt along all the various seals are advised to be replace during that mileage) the seals start to wear, then again hard to say with your driving habits or high usage of car (eg: mileage may be low, however you tend to idle in car waiting for people, your engine/belts etc are still running btw)
2) nope it is not, i have to get separately, and i just gotten the crankshaft oil seal, rm25 for original mitsubishi (proton part stockist, so i don't doubt it is fake.)
3) dust is the least of your concern, do take note that the engine takes up more beating and carbon from combustion than what mere dust can provide any harm. the only thing you need to be vary is probably how the mechanic applies their gasket/sealant, some of them even to this day uses the wrong method
4) parts alone atm i've spent 203 for rocket cover gasket, timing kit, crankshaft oil seal, water pump (since you're replacing timing belt, might as well replace the pump )
cam shaft oil seal shouldn't be too expensive i reckon since the crankshaft was 25. so around that figure, with labor in the 10-40? (it does take some time so labor wouldn't  be too cheap.)
at least half day work required, bearing should be included with the timing kit, unless its the aux belt bearings i think? will need to confirm that when im doing it much later (only preparing parts at the moment)

proton. i havent bought the aux/fan belts and camshaft oil seal though. and i gotten my parts from ck wong proton parts stockist which is also listed on proton official website btw
timing belt kit also 95 only. (including oil seal bearing and the belt)

=edit=
well, to be more accurate, water pump is ori gmb (no longer made in japan lolz) and crankshaft oil seal is mitsubishi, its 25 bucks for a small piece of plastic/metal/rubber seal thingy, so that shouldnt be fake as iinm other brands are in the rm5-10 price lol

someone selling for rm90 also:
https://forum.lowyat.net/topic/2766292