I would repot the healthy tree soon, after week or two, before new growth emerges, supposing, that spring is not far away in your location. But letting the tree still familiarize with changed environment and decrease its stress level after relocation from the store. And until repotting I wouldn't water it, unless the soil gets really too dry (I don't know your conditions, but usually it is better to keep your citruses on a dry side, than soggy). When repotting, I recommend to mix moist soil in advance, before putting the plant into the soil, and after planting maybe only add small amount of water (not flood it abundantly after planting, like it is common for repotting many other species). And it is useful to weigh the container with optimally moist soil, together with plant, so you know afterwards, how much water you must add when watering. I am using my local soil, not commercial mixes, so I can't recommend you some trade mark or product. I have very sandy soil here, I just mix some compost and manure in the local soil, sometimes a little bit slow mineral fertilizers too. I decide about fertilizing and watering by look of leaves, how healthy they are. When the soil becomes too dry, leaves start to curl slightly or turn their angle from their usual position. The color of leaves hints when there is need for some fertilizers. You should select well draining mix, still enough organics in it. Avoid mixes, where organics like peat or coco choir are dominating. And you should prefer liquid fertilizers (if your plant has fruits on, it needs some special treatment with fertilizers). But better use more diluted solution, than too strong. For your sick tree, I would not caused additional stress right now. Let the tree starts some growth first. As soon, as it starts to grow new leaves, treat it lightly with some diluted liquid fertilizer when watering. Repot after new growth is already strong enough. If this happens only when the weather is already unfavourable (there is a heat wave), then postpone repotting until late fall.