How to increment based on checkbox and input values - vue.js

Question

I am trying to create a button that will change based on how many checkboxes and input values are entered/selected.
For example, I have a button html that changes based on how many items are selected
Buy (0 items selected)
Buy 1 item (1 item selected/entered)
Buy x items (x items selected/entered)
using a buyCounter data variable set to 0 currently. I would like to increase/decrease this counter based on the checkbox/input values.
If a checkbox is selected, it counts as +1, if deselected -1.
On the input, it takes any value entered and adds its to the counter. For example if 4 is entered on an input, add +4 to the counter.
Currently not focused on if that number is more than the amount allowed, really just trying to figure out the counter part.
I created this example below in codepen to get close enough to what I'm trying to achieve. Any help in the right direction would be greatly appreciated
https://codepen.io/anon/pen/QooWZV?&editable=true&editors=101
TEMPLATE
<div id="app">
<v-app id="inspire">
<v-data-table
v-model="selected"
:headers="headers"
:items="desserts"
item-key="name"
class="elevation-1"
>
<template v-slot:items="props">
<td v-if="props.item.available > 1" style="display:inline-flex">
<v-text-field style="width:5%; padding-bottom:10px;"></v-text-field> of {{ props.item.available }}
</td>
<td v-else>
<v-checkbox
v-on:change="increment($event)"
v-model="props.selected"
primary
hide-details
></v-checkbox>
</td>
<td>{{ props.item.name }}</td>
<td class="text-xs-right">{{ props.item.calories }}</td>
<td class="text-xs-right">{{ props.item.fat }}</td>
<td class="text-xs-right">{{ props.item.carbs }}</td>
<td class="text-xs-right">{{ props.item.protein }}</td>
<td class="text-xs-right">{{ props.item.iron }}</td>
</template>
</v-data-table>
<v-layout row wrap class="">
<v-flex xs4 class="text-xs-left">
<v-btn
v-if="buyCounter < 1"
color="blue-grey"
class="white--text">
Buy
</v-btn>
<v-btn
v-else-if="buyCounter > 0 && buyCounter < 2"
color="blue-grey"
class="white--text">
Buy {{ buyCounter }} Item
</v-btn>
<v-btn
v-else-if="buyCounter > 1"
color="blue-grey"
class="white--text">
Buy {{ buyCounter }} Items
</v-btn>
</v-flex>
<v-flex xs4 class="text-xs-center">
</v-flex>
<v-flex xs4 class="text-xs-right">
</v-flex>
</v-layout>
</v-app>
</div>
SCRIPT
new Vue({
el: '#app',
data () {
return {
selected: [],
buyCounter: 0,
headers: [
{
text: 'Count',
align: 'left',
sortable: false,
value: ''
},
{
text: 'Dessert (100g serving)',
align: 'left',
sortable: false,
value: 'name'
},
{ text: 'Calories', value: 'calories' },
{ text: 'Fat (g)', value: 'fat' },
{ text: 'Carbs (g)', value: 'carbs' },
{ text: 'Protein (g)', value: 'protein' },
{ text: 'Iron (%)', value: 'iron' }
],
desserts: [
{
name: 'Frozen Yogurt',
calories: 159,
fat: 6.0,
carbs: 24,
protein: 4.0,
iron: '1%',
available: 1
},
{
name: 'Ice cream sandwich',
calories: 237,
fat: 9.0,
carbs: 37,
protein: 4.3,
iron: '1%',
available: 2
},
{
name: 'Eclair',
calories: 262,
fat: 16.0,
carbs: 23,
protein: 6.0,
iron: '7%',
available: 1
},
{
name: 'Cupcake',
calories: 305,
fat: 3.7,
carbs: 67,
protein: 4.3,
iron: '8%',
available: 1
},
{
name: 'Gingerbread',
calories: 356,
fat: 16.0,
carbs: 49,
protein: 3.9,
iron: '16%',
available: 3
},
{
name: 'Jelly bean',
calories: 375,
fat: 0.0,
carbs: 94,
protein: 0.0,
iron: '0%',
available: 1
},
{
name: 'Lollipop',
calories: 392,
fat: 0.2,
carbs: 98,
protein: 0,
iron: '2%',
available: 2
},
{
name: 'Honeycomb',
calories: 408,
fat: 3.2,
carbs: 87,
protein: 6.5,
iron: '45%',
available: 1
},
{
name: 'Donut',
calories: 452,
fat: 25.0,
carbs: 51,
protein: 4.9,
iron: '22%',
available: 1
},
{
name: 'KitKat',
calories: 518,
fat: 26.0,
carbs: 65,
protein: 7,
iron: '6%',
available: 3
}
]
}
},
methods:{
increment(event){
console.log(this.selected);
const isChecked = event;
if(isChecked){
this.buyCounter++;
}else{
this.buyCounter--;
}
}
}
})
Looking to create a button that recognizes how many items are about to be bought.