Have you shot wildlife before? And what sort of wildlife - animals, birds? Being brutally honest, if you can't use a DSLR on anything but full auto then you have however long 'soon' is to conquer a steep learning curve as to successfully shoot wildlife you need to get off auto mode. For stationary or slow moving animals/birds the 450D will cope. If you want birds in flight or charging lions, for example, you'd be better off with a newer camera with a better autofocus system and higher frames per second, but if your knowledge and technique isn't up to it, you'll still fail. Then there's the 75-300mm lens (the 18-55mm is far too short for wildlife unless it's big and right in front of you). You say it's pre digital but is it autofocus and how well does it suit the 450? If it isn't up to it then leave the DSLR at home. My advise would be:
Break out the 450 and long lens and the camera manual (download one if you don't have it). Probably the simplest settings for a beginner for wildlife are Manual mode (M on the top dial), aperture around f7.1 or 8 (you don't want the lens wide open) shutter speed as high as it will go in the light you have (I would suggest 1/600 or 1/800 minimum) and set the iso to auto. The camera will then use the iso to control the exposure. It's not the same as full auto as you are controlling the shutter speed and aperture. Use the centre autofocus spot only as it's the most sensitive on your camera. Ideally your focus point should be your subject's eye/s. You can play with the metering but I'd suggest evaluative if the 450 has it. That'll do for most situations so long as you're not shooting something pure white, pure black or shooting into the light (which is never great with wildlife anyway). Set servo autofocus and continuous shooting, that way you can fire off a few (slow!) frames by keeping your finger on the button. Practice. practice, practice.
However................. if you haven't done any of this before and this is a wildlife holiday then the danger is you'll get hung up on photography with a setup you're not used to when you might have been better just viewing and taking home the memories in your head. Maybe take both cameras, but limit the photography to when you are absolutely sure you can get a shot without stepping outside your capabilities. Then when you get home, if the wildlife photogaphy bug has truly bitten, do as Stewart says and upgrade the lens. The Tamron 70-300 has a good reputation without being expensive and will work well with the 450D. When you outgrow that think about the Tamron/Sigma 150-600, which is a whole new learning curve in itself. Only go for a new camera when you know that what you have is holding you back, as you'll almost certainly need something a step up from the xxxD range by then. The 750D has a much more advanced autofocus system (another learning curve) but still only manages 5 fps on continuous shooting, which is still fairly slow. To get better you need to spend a lot more money, which is why I'm still using a 550D with my Sigma 150-600mm............
Good wildlife photography is only partly about the kit you use. A big part is about you and how you use the kit. If you practice with the 450 and need some help, post some examples of your shots with all the settings you used and someone will always help.
Another thought - if the 450 hasn't been used much recently take a close look at the battery and maybe consider getting at least one new one. Driving a long lens, servo autofocus and lots of checking the previews on the screen (we all do it) will hammer the battery.
Oh, and shoot jpeg, not raw, otherwise continuous shooting won't be worth it, and tbh most wildlife shots don't need the additional data raw gives you.
