If you've only got the one gilt/sow she's unlikely to keep him from getting 'frustrated'
It's better to have at least 3 or 4 females to keep the boar happy. Is there no-one close who could loan you a boar until you've got enough breeding stock to keep him working all the time ?
You mention your other one had a problem with it's back end - was this as the result of injury or was it something in the breeding ? If it was something in the breeding - is your remaining gilt from the same litter ? If she's from the same litter with a possible problem - I'd be really wary of breeding from her to be honest
She's got to be able to take the weight of the boar at service (and it can be for as long as 30 minutes) and then you've got the pregnancy & farrowing to get through too - it might be too much for her to cope with.
If you do decide to go ahead and get him, I wouldn't put them straight in together. Keep him seperate from her for at least 3 weeks til you're sure he's not carrying any infections, then introduce them in neighbouring paddocks, after a day or so you can put them both in together - make sure there's plenty of flight space, if they don't get on and one has to leg it you don't want them jumping fences
Karen