I assume a "450 Off Road bike" will not be street legal? Maybe you can convert it to street legal in Spain? ... but you will have other, more serious problems using such a bike.

Most non street legal Off road bikes are basically ... Race Bikes. Motocross bikes mostly or hard Enduro bikes. Trust me, they don't make good travel bikes without doing A LOT of work ($$$$)

1. The long travel suspension will not work well two up and no passenger foot rests and missing LOTS of other things too.

2. Add luggage (but where? and How?) Race bikes have NO subframe.
With luggage mounted, suspension gets worse as you will have too much sag in the rear. Not good, will ruin steering geometry and handling goes to Hell.

3. Lighting. Race and Hard Enduro bikes have very little if any lighting and have very low Stator output. Not good for travel. You will not have good lighting or power to run heated gear.

What is your budget? $$$$
Probably should have started with this question first!

Newer bikes cost a lot of money. But if you buy a nice, low Km used dual sport bike, it could be a perfect project that you can actually make into a good travel bike.

A standard 350 to 500cc street bike could work for you too, but a Dual Sport bike will perform better off road, is stronger and if set up right, can be a comfortable and reliable travel bike.

You need to find a bike to adapt to soft luggage, has some sort of rear sub frame and is strong enough to carry passenger and your luggage.

Even though I spent months is Spain on two long bike trips, I only visited a few bike shops. I don't know what is for sale there now and don't know what older bikes might be available.

If your budget is low, I would try to find a nice old XT600 Yamaha. Will do everything you need if set up right, is super simple, air cooled and reliable. You can find them in the UK for sale, don't know about Spain.

There are a few others that could work. Look around.